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Dear Gini
Parenting & Lifestyle
Raising a Bilingual Child
There are a lot of myths surrounding language and kids. Should the child's mother tongue be the first language she learns? How about English or any other second language? When should that be introduced? Does introducing more than one language too early on confuse and delay the child's progress?
When my daughter was 7 months old, we hired a lady to help around the house as well as to help look after her. She was Zimbabwean. Having her around was a godsend but it meant we had to speak to G only in English throughout the day. I didn't mind it. Both my husband and I are comfortable enough in the language to not find using it cumbersome with our baby. Speaking to her in English didn't involve too much effort on our part so we didn't think much of it. But when we relocated to India, problems slowly began cropping up.
G was an early talker. At the age of 15 months, when we moved back, she had a decent vocabulary. Although she couldn't string sentences, she could happily use the right words to convey her needs. I don't know why the question of language started cropping up in conversations with family then. Seeing that everyone in the immediate family is proficient in the language, it was probably more of a sentimental thing than a practical one. My parents, in laws, distant relatives all pointed out the fact that G couldn't speak a word of Assamese, which is her mother tongue. It didn't bother me, initially. But it built up. I began to wonder if my daughter too was going to turn out like those kids who can't speak the language that their parents converse in. I began worrying if I should make a conscious effort to teach her the language. If so, when? Wouldn't it make her forget English? Conversely, there were also times when I brushed off these worries with the firm faith that since she is exposed to the language on a daily basis, she was bound to pick it up. Sooner or later. But the worries would always come back and haunt.
Today, G is 2 years and 8 months old. Since the last month, she has been speaking both languages fluently. She switches between languages with ease and doesn't seem to be confusing words of one with the other. She has her own way of reserving one language for a particular situation or a particular person. But that doesn't mar her proficiency. Not only has she not forgotten English but is also beginning to pick up phrases in a third language, Hindi.
I don't want to sound boastful. I have no reason to as whatever strides she has made have come of her own accord. I just want other parents out there to know that in my experience, raising a bilingual child is easier than it is made to sound. If they are exposed to more than one language from an early age, they are likely to pick both up automatically. So, if you have a child between 1 and 2 years of age and are wondering which language to teach her, my advice would be to stop 'teaching'. Instead, converse with her in both languages. Read to her a lot. Point out things and tell her what it is called in both languages and before you know it, she will be chattering away in both. Bear in mind, though, that children speak at their own pace so don't compare or be disheartened if it takes your child longer than you expected. Happy conversing!
Published by Dear Gini
My name is Mousumee and I stumbled into the world of parenting while I was looking elsewhere. Predictable! I had been married for just a few months when I moved to Cape Town to set up my first home with my husband. We had plans. Plans of traveling, of going on thrilling, sometimes reckless adventures. But we did something quite else. We made a baby! With our families in another continent, we have learnt everything on the job. From choosing car seats to choosing a name, from the ups of giving birth according to plan to the downs of having an emergency immediately afterwards, the proverbial roller-coaster has become our world. Welcome to it! Motherhood, as I was told, is a wonderful journey. But often, it is lonely. This blog is my place to talk about it. So come on, let's talk! Mousumee x	View all posts by Dear Gini
bangalore, bilingual, early talker, language and kids, learning a language, learning to speak, love, mother tongue, Motherhood, parenting, raising a bilungual child
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Q: Unknown encoding format when reading data from USB COM port using Trimble Pathfinder ProXRT receiver I'm using a Python script to read data from the USB com port and write it to a file. This is a snippet from the same.
enter code here
def read_gps():
  ser = serial.Serial()
  ser.port = "COM3"# "/dev/ttyUSB0"
  ser.baudrate = 38400
  # 19200

  ser.open()

  ts = str(time.time()).split('.')[0]
  gps_file = open("./proxrt_gps_input_"+ts+".txt", 'w')
  line_cnt = 0
  stop_cap = False

  # Read from serial port and write to file till stopped
  while (1):
    cur_pos  = ser.readline()
    line_cnt += 1

    print(cur_pos, file=gps_file)

The script with an older Trimble receiver used to write something like this to the output file in the NMEA format,
$GPGGA,013249,2938.6513,N,08220.7101,W,0,00,0.0,26.2,M,30.1,M,015,0000*52
Currently, with the ProXRT receiver, it writes lines similar to the following,
b'\x00\x03\x00\xe3\x00V\x00\x00\x00\x0f\x00\x03\x00\xd0\x01e\x00\x00\x00\x11\x00\x03\x00\xeb\x00\xd5\x00\x00\x00\x13\x00\x03\x00\xda\x00\xda\x00\x00\x00\x0c\x00\x03\x00\xb9\x00\xa9\x00\x00\x00\x0e\x00\x03\x00\xe6\x00\x91\x00\x00\x00\x87\x01\x03\x005\x00\xc8\x00\x00\x00\x8a\x01\x07\x00-\x00\xdf\x8d\x00\x00\xd1\x03\x02(@\xf9J\x00\x01\x10\t\x14f\xfe\x9c\x08\'\x00\x00\x00\x0c&\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x17\x19\x03\x83\x0c\x00\x00\x00\x00\x00\x00\x0b\x13\x00\x00\x00\x00\x00\x00\x8a\x1b\x00\x00\x00\x00\x00\x00"\x19\x1c\t\x02\x03\x00*\x01V\x00\x00\x00\x08\x00\x03\x00M\x00~\x00\x00\x00\x17\x00\x03\x003\x00\xcb\x00\x00\x00\x13\x02\x03\x002\x00`\x00\x00\x00\x01\x02\x03\x00\xa6\x00\x00\x00\x00\x00\x06\x02\x03\x00\x0e\x00\xcf\x92\x00\x00\x03\x02\x03\x00\xcb\x00L\x00\x00\x00\x12\x02\x03\x00\x19\x00\'\x00\x00\x00\x04\x02\x03\x00\xd5\x00\x84\x00\x00\x00\x11\x02\x03\x00\xf4\x00\x0e\x00\x00\x00\n'

b'\x02\x03\x00\x02\x01$\x00\x00\x00\t\x00\x03\x00:\x00\xfa\x00\x00\x00\x83\x01\x07\x00&\x00\xea}\x00\x00\x85\x01\x03\x00\x1c\x00\xf5\x00\x00\x00\x01\x00\x03\x00\x03\x00\xa8\x00\x00\x00\x0b\x02\x03\x00\xeb\x01\x12\x00\x00\x00\x10\x00\x03\x00\x16a\x03\x02(@vJ\x01\x01\x002\x00\x00\x00\x0e\x02\x03\x00\xd8\x00z\x00\x00\x00\x10\x02\x03\x00\x1d\x00?\x00\x00\x00\x0b\x00\x07\x00\x1b\x00\xa9\xa4\x00\x00\n'

Tried decoding it thinking it was ASCII but it gives a weird output. I just want to get the latitude and longitude from the receiver in either Python or C++.
I am new to using these tools and formats.

A: I recommend downloading a manual for the ProXRT - do a google search for "trimble proxrt user manual" and choose a link from the Trimble.com site for User Guide - the PDF manuals for firmware 4.44 or 4.71 should be suitable (I don't think operation changed much).
One caution is that NMEA Output was an optional extra for ProXRT receivers - it was common but not all ProXRT receivers were sold with this enabled.  And if you don't have the NMEA output option installed, no method is going to turn it on (unless you buy the NMEA Option and I'm sure that will cost a silly amount).
I've given two methods below - the first is much easier but it will only work with a recent firmware version (I guess firmware earlier than 5.10 will not work and I suspect it will not work on units labelled ProXRT, only on those labelled ProXRT2).  The second approach is more general, but it requires a network connection, and for that you need the ProXRT Multiport Adapter.
First step - reset to factory settings
Before you do anything else, reset the ProXRT to factory settings.  To do this, turn the ProXRT on, and once it has started up (the screen will show an SV count and power level, etc), press and hold the power button for about 30 seconds (a 5 second press turns the receiver off, a 15 second press clears all satellite data, a 30 second press Clears the FS = File System which resets it to factory defaults.)
Method 1 - Use Trimble GNSS Status to connect to ProXRT via Bluetooth and set up NMEA on a Bluetooth serial port
This will only work with recent firmware, probably only on ProXRT-2, and can only set up NMEA on Bluetooth, not the physical serial ports.
Because this method depends on Bluetooth, you should be aware that ProXRT has truly terrible Bluetooth - try to have the ProXRT within 1M of your PC, preferably in an environment with low RF noise!

*

*Start the ProXRT

*On the PC where you are going to run your script, search for the ProXRT on Bluetooth (it will usually be listed as "ProXRT, 1234K56789: SomeName" where the number in the middle is the serial number) and pair with it.  The default Bluetooth pairing code is 0000 (four zeroes)

*Under Settings / Devices / Bluetooth and other devices, click on "More Bluetooth options"

*A "Bluetooth Settings" dialog box opens, with 3 tabs.  Click on the "COM Ports" tab.

*Look through the list of ports to find the ProXRT, make a list of the Port numbers in the left-hand column which correspond to "ProXRT, 1234K56789: SomeName 'COM1' ", "ProXRT, 1234K56789: SomeName 'COM3' ", "ProXRT, 1234K56789: SomeName 'COM3' "

*Search for the GNSS Status application on the Trimble.com web site, download this and install on your PC

*Run GNSS Status, select Source, under Position Source, select Bluetooth

*The ProXRT (and any other Bluetooth devices which are nearby) should be listed - select the ProXRT and click Select

*If GNSS Status connects, wonderful.  If not, try a few more times then give up - your ProXRT is probably not compatible with GNSS Status

*Assuming GNSS Status connected to the ProXRT. . . .

*Select the GNSS Status NMEA page

*Select the NMEA sentences you want to use - for your example select just GGA

*Click on the Save button (not essential but then GNSS Status will remember your NMEA selection for next time)

*Click on the Apply button

*GNSS Status will give a list of ports - probably "Bluetooth SPP1 (Connected)", "Bluetooth SPP2", "Bluetooth SPP3"

*Select Bluetooth SPP2 - GNSS Status will tell you that the settings have been applied

That's it for the setup, you can exit from GNSS Status, ProXRT will remember its configuration even if you turn it off then on
Now update this line in your script ser.port = "COM3"# "/dev/ttyUSB0" so it refers to the Port number corresponding to "ProXRT, 1234K56789: SomeName 'COM3' " (see the 4th step above)
Run your script - GGA should come through on Bluetooth (which is unreliable on ProXRT, as I said)
Method 2 - Use the ProXRT WebUI via network
This is a more general solution than Method 1.
I have a suspicion that some ProXRT were shipped with WebUI disabled, making this unuseable.  I'm not sure how you'd know except by trying it.
And this method requires the ProXRT multi-port adapter to give a network port.
Here goes:

*

*Connect the ProXRT to the same network as your PC (or if your PC is not on a network, you may be able to connect a network cable directly between the ProXRT and your PC, most modern network adapters will handle this)

*Press the up-arrow on the front of the ProXRT until it displays the IP address for the ProXRT.

*If the IP address is 0.0.0.0, wait a bit - at most a few minutes.  If it doesn't give a non-zero value you probably don't have a DHCP server on the network, try adding a network Hub / Network Switch

*If you still don't manage to get an IP address, maybe the ProXRT doesn't have network ability (I thought they all had but I'm not sure).  Or the multi-port adapter is damaged.  Or something else.  Hard to diagnose this without swapping out bits.

*Now you've got a network connection, fire up a browser on your PC (I use Edge or Chrome, other browsers should also work)

*In the browser address line, enter the IP address displayed by the ProXRT then hit Enter

*You should get a Trimble splash screen, then it will ask for username and password.  Because we did the factory reset, I expect the default settings with the user "admin" and the password "password".

*Now we've got to the "WebUI", click on the I/O Configuration label, this will give a list of ports and say what is enabled on each port

*Because you reset the ProXRT to factory settings, no messages / data types should be listed as Input or Output for any of the ports

*If you are using the round "Lemo" connector for serial, click on the line for "Lemo"

*If you are connecting using the multi-port adapter, click on the "Modem 1" line

*The top-right drop-down list allows selection of protocol (CMR, RTCM, RT17/RT27, NMEA, etc.) - select the NMEA option

*If you see some of the other options I've listed but not NMEA, this means the ProXRT doesn't have the NMEA option, so stop right there, NMEA is not available!

*On the NMEA screen, confirm the Baud rate and Parity match what you want ("38400" & "N" from your script sample)

*Select 1Hz for GGA

*Click OK
In the IO Configuration screen, it should list "NMEA-GGA(1Hz)" against Lemo or Modem 1

That's it for the setup, you can close the WebUI and disconnect the network cable from the ProXRT, it will remember its configuration even if you turn it off then on
Your script should now be able to see NMEA from the ProXRT serial port
NOTE:  If you use WebUI to configure Bluetooth ports, turn on the "maintain configuration when connection dropped" checkbox otherwise the ProXRT may discard your settings

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The city of Melbourne divides.
It's the Vixens and the Magpies. It's #TheBattle.
On Saturday April 28, the two Victorian teams will launch the 2018 Suncorp Super Netball (SSN) season, as they battle it out in round one action at Hisense Arena.
Last year, the Vixens proved too strong, claiming two victories over the Magpies.
But this year, we're ready to FLY. We're MADE TO FLY. And we've got our eyes set on the Vixens in round one.
Come watch the Magpies take flight in 2018.


A private 4,366-square-foot, centrally air conditioned estate with four bedrooms and 4.5 bathrooms, this Estate Home is situated on the 8th green of the island links golf course and offers stunning views of Winding Bay and the pristine 2 -mile beach from almost every room in the house.
A wonderful flowing floor plan draws you into the great room of this beautiful estate home with a dramatic vaulted ceiling and a wall of windows. French doors lead out to the expansive covered decks on both sides of the house. You'll enjoy breathtaking views of the beach, golf course and Winding Bay through one window and the famous fishing Marls of Winding Bay through another window. It's the perfect spot to enjoy the tropical surrounding.
Colorful, laid-back island influenced fabrics, furnishings and artwork, create a warm and welcoming home. The upper level offers a spectacular great room, a well-equipped open custom kitchen, complete with an entertaining island with granite counter tops and barstool seating, where guests can enjoy their favorite rum cocktail. The master bedroom with an en-suite bathroom is also located on the upper level, has a king bed and a separate study/office area. Also located on the upper level is one of the additional guest suites with a king bed and private bathroom.
The lower level has two bedrooms, each with private baths that open onto a covered patio and pool deck. With a bunk bed in one bedroom and two twin beds in the other, all the kids in the family can enjoy themselves in this comfortable home. Just outside the bedrooms is a spacious entertainment area, complete with a pool table and ping-pong table for friendly family competition. A wet-bar, laundry room, garage and large storage area can also be found on the lower level of this gracious home. The beautiful native landscaping keeps the pool area very private while still creating picture perfect views of the golf course and bay just beyond.
This private estate is easily accessible to all the amenities of The Abaco Club on Winding bay: infinity edge pool, day dock, tennis courts, golf course, beach and dining facilities. Golf carts are used on the property for convenient transportation. A special home in a special place on Great Abaco Island.


We offer the most programs and class times to fit everyone's budget and schedule.
Depending on the program you prefer, cost can range anywhere from $40 to $90 per month.
programs might work best for you.
We Never Ask You to Sign a Membership Contract!
NOT Because You HAVE To!!!
point during the contract length, or can't continue, for any reason, you will still need to pay each month until the contract expires. This can be anywhere from 6 mos to a year. We do not believe in obligating our students to train with us. We want you with us because YOU WANT to be.


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Posted inBasketball, High School Sports, Sports
Bangor girls basketball team avenges loss to Mt. Blue
by Dave Barber January 29, 2013 January 29, 2013
BANGOR, Maine — As the Mt. Blue Cougars poured in 3-pointers in the first half of Tuesday night's game at Red Barry Gymnasium, Bangor girls basketball coach Katie Herbine knew she had to make a change.
"I hesitated going to the 50 defense a little bit," said Herbine. "That's our man-to-man."
She couldn't afford to let the Cougars keep hitting open 3s, though.
The resulting hounding of the Cougars propelled the Rams to a 55-43 victory.
"I challenged the kids at halftime," said Herbine. "They answered in the second half."
Bangor, now 12-3, clamped down and held Mt. Blue (10-5) to a single 3 in the second half, and that came in the first minute-and-a-half of the third quarter.
The Cougars held a 34-27 lead at that point, their biggest of the game.
Mary Butler took charge then, hitting three straight Bangor baskets within a minute and assisting on a Jordan Seekins hoop while Mt. Blue was only able to add a Miranda Nicely field goal.
Bangor's Sarah Bragg added the last two of her eight points with 4:12 left in the third to put the Rams ahead for good at 37-36.
Bangor pushed the margin to five points at the end of the third and limited Mt. Blue to three points in the final quarter on a free throw in the opening minute and a last-second field goal.
"They threw everything they had at us," said Mt. Blue coach Tom Philbrick. "Tonight was their night."
The Cougars, ranked No. 2 in the Heal points entering the game, fell behind 6-0 before they started hitting 3-pointers on the Rams' zone defense. Mt. Blue made eight in the first half, including three by Nicely that got their scoring untracked.
Bangor changed to its man-to-man defense in the second half, took the easy 3-point shots away from the Cougars and took Mt. Blue out of its uptempo game.
"We started standing still," said Philbrick. "We weren't moving without the ball. I attribute that to Bangor."
Butler said the Rams' defense triggers their offense. They play better when they can get into their uptempo game, she said.
"We came out ready [in the second half]," said Butler, who finished with a game-high 18 points. "Our intensity increased as the game went on."
Seekins posted 11 points for the Rams, and Cordelia Stewart added 10 points and a strong defensive presence under the basket, including back-to-back blocks midway through the fourth. Bragg finished with eight points.
Nicely hit four 3-pointers for Mt. Blue and registered 16 points, and Gabby Foy added 11.
Another bright spot for the Rams was their free-throw shooting, said Butler.
Bangor hit 6 of 7 free throws in the final 1:16 to put the game away. They were 12 of 13 (92 percent) for the game.
"The time and effort we've put into it [in practice] have definitely paid off," she said.
Herbine pointed to Butler's rebounding as well as her and Stewart's efforts on both ends of the floor.
"When Mary and Cordelia are working well together, they're fun to watch," she said.
Previous Orono officials looking into new mold complaints from renters at The Grove
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Online retailers have a number of incentives to start offering free shipping to their customers. Free shipping is an excellent way to increase your website's conversion rate and strengthen relationships with your customers, which can lead to an increase in customer Lifetime Value (LTV). Consumers are used to free shipping with websites like Zappos and are even paying for memberships with Amazon Prime and FreeShipping.com to get the benefits of free shipping. As such, they see shipping fees as unnecessary and may even consider inflated shipping rates to be a scam. This infographic lays out the main reasons why online retailers should consider offering free shipping, or partnering with a service such as FreeShipping.com to help defray shipping costs.
Learn why you should offer free shipping to your customers.
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Improve your online reputation while you increase sales volume and profitability.
Avoid Complaints: Consumers expect free shipping and shop elsewhere when it's not available.
Free shipping minimums are an enticing way to increase Average Order Value.
In 2012, 20% of customers said that free shipping is the most important factor in where they shop online.
Your great service will be discussed on comparison shopping websites, shopping forums, and directories, rather than on scam sites.
63% of buyers said that free shipping has led them to recommend a retailer.
61% of consumers said they will cancel their purchase if free shipping isn't offered.
People shop with confidence when their risks are low.
Companies that adopted free return shipping reported a 58% – 537% increase in sales over two years.
Build loyalty by giving customers points towards free shipping or offering a service similar to Amazon Prime.
Offer free shipping on high-margin items or the house brand to make more money per order.
Posted in Ecommerce 101, Infographic. Tags: Ecommerce business, ecommerce marketing, ecommerce tips, ecommerce tips 2013, free shipping.


Although the fashion weeks have been over except over here. But if I was going to be invited by all the coveted designers what I would have worn.
So that all the street style photographers would have been flocking around me… I would walk from here to there like Im in such a hurry and "Oh No! I cant keep a show waiting" sort of attitude muhahaha ha ha.
For Milan Im going with a Chic and Classy with a twist.
Smooth stacked rings, for this look I will add a lot of gold bangles and braclets.
Well this was round-up for a Fashion week dressing… do let us know your views and feedback in reply.


text:
About BIG
General terms and conditions of Big Machinery B.V.
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way that the provision may be reasonably invoked by Big Machinery against the counterparty. If the court decides that one or more provisions from these general terms and conditions is unreasonably onerous, it will not affect the effectiveness of the other provisions.
If Big Machinery uses a non-Dutch version of these general terms and conditions and there are differences between the Dutch version and the non-Dutch version, only the Dutch version will be binding.
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All offers are non-binding, unless otherwise indicated in writing. Agreements will be established when we send a written or electronic confirmation of the agreements or start with the implementation of the agreement after acceptance of our offer or any (partial) payment.
All our proposals are open for a maximum of 3 days, unless this period is deviated from in writing. All prices indicated therein exclude VAT and are in Euro (unless another currency has been indicated) and are non-binding in case of an interim increase in cost-determining factors.
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Agreed delivery terms are never deadlines and are indicative only. The delivery period will start after the counterparty has fulfilled all its obligations. If delivery fails to take place due to a circumstance not attributable to Big Machinery, the risk of the to be delivered good will transfer to the counterparty from the moment this circumstance arises. If delivery fails to take place due to a circumstance attributable to Big Machinery, the counterparty must send
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Unless otherwise agreed, payments must be made within three days after the invoice date. Big Machinery is entitled to request one or more advances for the payment of invoices.
In the event of late payment, the counterparty will be legally in default after expiry of the payment period specified above, without notice of default being required. The counterparty will in that case owe Big Machinery a default interest of 1% per month (or part of a month) from the due date.
If the counterparty fails to completely, timely or properly fulfil one of more of its obligations under the corresponding agreement(s), it will owe Big Machinery the (to be) incurred costs to determine the liability and recovery of the claim, which (extra) judicial costs will be set at 15% of the principal amount with a minimum
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The ownership of the goods delivered by Big Machinery will transfer to the counterparty if all amounts claimable by Big Machinery from the counterparty have been fully paid, even if these arise from other deliveries and/or damage. Until the moment of the transfer of ownership, disposal or encumbrance of the
goods are forbidden, as well as providing them as security to third parties. This provision is effective in accordance with property law.
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Any claims related to invoices must be submitted in writing to Big Machinery within fourteen days of the invoice date on pain of forfeiture. Goods will be delivered without any warranties ("as is"), unless expressly agreed otherwise in writing. Complaints related to the quality of the goods delivered for sale, to the extent an express written warranty has been given, will only be accepted by Big Machinery if they have been submitted in writing within five days after they have been found or reasonably should have been found by the counterparty, on pain of forfeiture of such right. Any provided warranty will only be applicable after full payment of the invoice of Big Machinery.
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text:
NobleBorn Prince Daniel of Moscow
Commemorated on August 30, March 4
Holy Nobleborn Prince Daniel (Daniil) of Moscow was born at Vladimir in the year 1261. He was the fourth son of Saint Alexander Nevsky (Comm. 30 August and 23 November) and Righteous Vassa. Two years after birth he lost his father. The date of his mother's repose is not indicated in the chronicles; it is known only, that she was buried in the church in honour of the Nativity of Christ at the Vladimir Uspenie monastery (the Princess monastery), and the people in the surroundings venerated her as "Righteous" ("Pravedna").
In 1272 holy Prince Daniel received as his allotted portion the city of Moscow with its adjacent lands. The holy prince built on the banks of the River Moskva (Moscow) a church (and alongside it a monastery) in honour of his same-name patron saint, the Monk Daniel the Pillar-Dweller (Comm. 11 December). The Moscow principality was during this period small and unobtrusive. While growing up, holy Prince Daniel strengthened and expanded it, not in manners unjust or coercive, but instead benevolent and peace-loving. In Rus' it was a time of unrest. Fratricidal strife amongst the appanage princes was rife. And often, thanks to holy Prince Daniel, and his incessant striving for unity and peace in the Russian Land, bloodshed was averted. In 1293 his brother, the Great-prince Alexander Alexandrovich, together with Tatars summoned from the Horde and headed by Diuden ("the Diudenev Host"), laid waste to Russian cities: Murom, Suzdal', Kolomna, Dmitrov, Mozhaisk, Tver'. Prince Daniel decided to adjoin them to Moscow, to save their people from perishing. There was not the strength for resistance. Together with his people, the prince braced himself for terrible destruction and pillaging. Standing up for his rights, Saint Daniel was compelled to come out against his brother near a place, called Yur'evo Tolchische ("Yur'evo Threshing-Mill"), but here also the yearning for peace won out in him, and bloodshed was averted.
In 1300, when the Ryazan prince Konstantin Romanovich, having summoned Tatars to his aid, was occupied in secret preparations for a sudden assault on the lands of the Moscow principality, Prince Daniel went with an army to Ryazan, and beating the enemy, he took captive Konstantin and destroyed a multitude of Tatars. This was a first victory over the Tatars, though not a tremendous victory, but it was noteworthy nonetheless – as a first push towards freedom. Having beaten the Ryazan prince and scattered his confederates the Tatars, holy Prince Daniel did not take advantage of his victory to seize foreign lands or take booty, as was the accepted custom during these times, but rather he displayed an example of true non-covetousness, love and fraternity. The holy prince never resorted to arms to seize the lands of others, nor did he ever snatch away the property of other princes either by force or by treachery. And for this the Lord saw fit to expand the boundaries of his princely realm. Ioann Dimitrievich, prince of Pereslavl'-Zalessk, a nephew of Daniel, was gentle and pious and benevolent towards the poor, and he esteemed and loved his uncle; dying childless in 1302, he bequeathed his principality to Saint Daniel. The Pereslavlsk lands together with Dmitrov were, after Rostov, foremost in number of inhabitants, with corresponding fortification befitting a major city. Pereslavl'-Zalessk was well protected on all sides. But the holy prince remained faithful to Moscow and did not transfer the capital of his princedom to the stronger and more significant seat of the Pereslavl' of this period. This annexation moved Moscow up to be numbered as the most significant principality. And here was set in place the principle of the unification of the Russian Land into a single powerful realm.
How wondrous over the expanse of ages was clearly manifest the Providential Will of God concerning the Russian Land and its destiny!
Grateful in remembrance of the constant Blessing of the Hodegetria ("Way-Guide Mother of God) both in his personal life, and also in the life of the Russian realm, Saint Daniel's father – Saint Alexander Nevsky, had expressed it in the words: "God is not in might, but in right!".
In 1303 Saint Daniel fell seriously ill. He assumed the monastic great-schema and commanded that he be buried at the Danilov monastery. Through deep humility he wanted to be buried not within the church, but in the common monastery cemetery. The holy prince died on 4 March.
Within the passage of less than 30 years after the repose of holy Prince Daniel, the Danilov monastery founded by him was transformed into the Moscow Kremlin, the church was transformed into a parish church, and the cemetery became non-monastic. During the time of Great-prince Ivan III (1462-1505), the Monk-prince Daniel gave reminders of himself to his forgetful descendents. As a stranger he appeared to a youth attendant on the great-prince and said: "Be not afraid of me – I was a Christian and the master of this place, my name is Daniel Prince of Moscow, and by the will of God I am here. Tell about me to Great-prince Ioann (Ivan) saying: thou delightest thyself while yet having forgotten me, but God hath not forgotten me". And after this it was that the great-prince established the singing of cathedral panikhidas for his ancestral princes. During the time tsar Ivan the Terrible, at the grave of Saint Daniel was healed the dying son of a barge merchant. The tsar, struck by the miracle, renovated the ancient Danilov monastery and established a yearly church procession, made by the metropolitan to the place of burial of the holy prince, serving there a panikhida.
In 1652 holy Monk-prince Daniel was glorified with the uncovering of his incorrupt relics, which on 30 August were transferred to the church in honour of the Holy Fathers of the Seventh OEcumenical Council.
The holy relics were placed in a reliquary "to the glorifying of the Holy Trinity and for the healing of the infirm". The Moscow metropolitan Platon (+ 1812), in the Vita of the holy prince compiled by him, writes: "This original founder laid the foundation of present-day majestic Moscow, going about this with quiet steps upon a small foot-path. And thus as with any edifice, built not with extreme haste but the rather instead with great artifice and skill, doth receive a particular solidity and doth stand indestructible for a long time; and just as a tall tree growing for many a century, and having started first of all with a small sprout, and thickeneth little by little, with its branches spreading about far around, so also was it needful for this city to grow from the small, but solid root, in order that its first glimmer not beshadow the eyes of the envious, and that initially it not be disturbed or felled early on, but rather grow up to its true height. Thus did this founder prepare the great city given him; though small, but shining uninterrupted by any wafting of the wind, he did bequeathe the great glory of its rise to his son Great-prince Ioann (Ivan) Danilovich, called Kalita".
© 1996-2001 by translator Fr. S. Janos.


text:
\section{Introduction}

\begin{figure*}
    \begin{center}
        \includegraphics[width=17.8cm]{FigA.png}
    \end{center}
    \caption{\label{FigAllStructures}(Color online) Phases of oxidized Ag(111) observed in the present
    study. Further phases observed previously are specified in
    Table~\protect\ref{TableConditions}. The STM images to the
    left in the panels are supplemented by the FFTs of the STM
    images (middle parts of the panels) and by the theoretical
    LEED images simulated on the basis of the symmetry
    assignements (right-hand parts).
    The surface directions for all images are indicated in panel (a).
    (a) $70 \mbox{ \AA} \times 70 \mbox{ \AA}$, 1.12~nA, 9.8~mV.
        The depressions
        are interpreted as chemisorbed atomic oxygen with a coverage of approximately 11\% of a monolayer.
    (b) $200 \mbox{ \AA} \times 200 \mbox{ \AA}$, 0.62~nA, 39.1~mV.
        $p(4 \times 5 \sqrt{3})$rect.
    (c) $200 \mbox{ \AA} \times 200 \mbox{ \AA}$, -1.2~nA, -92.8~mV.
        $c(3 \times 5 \sqrt{3})$rect.
    (d) $200 \mbox{ \AA} \times 200 \mbox{ \AA}$, 0.99~nA, 39.1~mV.
        $p(4 \times 4)$.
    (e) $200 \mbox{ \AA} \times 200 \mbox{ \AA}$, 0.64~nA, 110.5~mV.
        $c(4 \times 8)$.
        The inset shows the same surface phase measured with a different tip state.
        $60 \mbox{ \AA} \times 60 \mbox{ \AA}$,
        -0.34~nA, -109.6~mV.
    (f) $200 \mbox{ \AA} \times 200 \mbox{ \AA}$, 0.41~nA, 1250,0~mV.
        Stripe structure.}
\end{figure*}

\begin{figure}
    \begin{center}
        \includegraphics[width=8.6cm]{FigB.png}
    \end{center}
    \caption{\label{FigStructuresAtthesametime}(Color online)
    Coexistence of phases.
    $250 \mbox{ \AA} \times 250 \mbox{ \AA}$ STM image of a Ag(111) sample exposed to $5 \times 10^{-8}$~Torr
   atomic oxygen for 40 min at 500 K sample temperature (-0.44~nA, -55.8~mV).}
\end{figure}

The oxidation of the Ag(111) surface is a fascinating example of
how a seemingly simple system can escape a detailed understanding
for decades, in spite of repeated and thorough efforts. The
history of surface science investigations of the oxidation of
Ag(111), briefly reviewed recently,\cite{Michaelides2005} started
back in the sixties and early seventies with a couple of single
crystal studies\cite{Mueller1966,Degeilh1969,Dweydari1973} and
took a serious upswing with the investigations by Rovida
\textit{et al.} in the seventies,\cite{Rovida1972,Rovida1974} who,
in particular, reported on the now renowned $p(4 \times 4)$ phase.
In the following much of the effort was directed towards this
phase,\cite{Albers1977,Campbell1985,Bare1995,Bukhtiyarov1999,Carlisle2000a,Carlisle2000b,Li2003,Li2003a,Li2003b,Michaelides2003,Bocquet2003a,Bocquet2003b,Michaelides2005,Schnadt2006,Schmid2006,Reichelt2007}
but also numerous other studies of more general character
appeared,\cite{Engelhardt1976,Albers1977a,Albers1977,Benndorf1983,Grant1984,Campbell1985,Schmeisser1985,Campbell1986,Spruit1989,Spruit1989a,Wang1991,Reijnen1991,Bao1993,Pettinger1994,BuatierdeMongeot1995,Raukema1996,Bao1996,Lacombe1996,Rocca1997,SchedelNiedrig1997,Bukhtiyarov1999,Li2002,Li2003,Michaelides2003,Stegelmann2004,Reichelt2007,Reicho2007,Reichelt2007a}
as well as works concerned with the reactivity of O/Ag(111)
towards, in particular, the oxidation of ethene and methanol.
\cite{Campbell1985b,Campbell1985c,Tan1987,vanSanten1987,Hawker1989,Mukoid1990,Wu1994,Carley1998,Boronin1999,Avdeev2001,Scheer2002,Stacchiola2001,Klust2006,Gao2007,Gomes2007,Greeley2007,Zhou2008,Huang2008,Christopher2008}
Only recently the atomic-scale geometric model for the $p(4 \times
4)$ phase was completely revised on the basis of a combination of
scanning tunnelling microscopy (STM), surface x-ray diffraction
(SXRD), and x-ray photoemission spectroscopy (XPS) experiments and
density functional theory
calculations.\cite{Schnadt2006,Schmid2006} Subsequent, detailed
low-energy electron diffraction and CO reaction studies supported
this atomic model.\cite{Reichelt2007a,Klust2007} At the same time
an increasing number of studies have indicated that the phase
diagram of the oxidized Ag(111) surface is more complex than
previously anticipated. While earlier studies only stipulated the
existence of the $p(4 \times 4)$ and an atomic adsorbate layer
(including a phase with a local
$p(\sqrt{3}\times\sqrt{3})$R$30^\circ$ symmetry), recent studies
have reported phases containing both less\cite{Reicho2007},
equally much,\cite{Carlisle2000b,Schnadt2006,Reichelt2007} and
more\cite{Schnadt2006} oxygen than the $p(4 \times 4)$ phase (cf.
Table~\ref{TableConditions}). Many of the reactivity studies have
assumed that the catalytical properties of Ag are associated with
the $p(4 \times 4)$ structure due to the anticipation that this
phase should be predominant under oxygen pressures similar to
those under reaction conditions.\cite{Li2003b,Li2003a} These
investigations have in general not taken into account the
existence of further phases and, hence, a much more complex phase
diagram for the Ag-O system will fundamentally change our picture
of what happens in the catalytic process. At present, growing
evidence exists that the static phases found in surface science
studies may neither be responsible for the surface's catalytic
activity itself nor be preserved during catalytic
operation.\cite{Stampfl2002,Hendriksen2004,Ackermann2005,Reuter2006,Reichelt2007}
The surface is rather to be considered as a dynamic medium, the
structure of which changes in response to the changing chemical
environment. The formation and breakup of these static Ag-O phases
are still of significant interest due to the phases' role in the
overall dynamics of the catalytic process. Thus surface science
studies with their unprecedented ability of clarifying the
atomic-scale structure still retain their value and validity.

Here we present a detailed and thorough experimental and
theoretical study of oxygen adsorption on Ag(111), with data
concerning both previously observed structures and hitherto new
unobserved phases of oxidized Ag(111). The present study shows
that the $p(4 \times 4)$ phase is embedded in a wider context of
two additional phases, the $c(3 \times 5 \sqrt{3})$rect and $p(4
\times 5 \sqrt{3})$rect structures. It is also found that the $p(4
\times 4)$ structure, to our knowledge in contrast to the $c(3
\times 5 \sqrt{3})$rect and $p(4 \times 5 \sqrt{3})$rect phases,
can host foreign atoms and molecules. Finally, the existence of
two more phases with a $c(4 \times 8)$ and "stripe" character is
reported.

\section{Experimental and theoretical methods}

The experiments were carried out in the STM laboratory and at the
vacuum ultraviolet/soft x-ray synchrotron radiation facility
ASTRID at Aarhus University. In the STM laboratory we used an
ultra-high vacuum (UHV) chamber with a base pressure of $1 \times
10^{-10}$ Torr, equipped with standard instrumentation for sample
cleaning, the Aarhus STM,\cite{Laegsgaard1988} a low-energy
electron diffractometer, and a thermal gas cracker from Oxford
Instruments for atomic oxygen exposure with a cracking efficiency
which in our experiments varied from 30\% at $10^{-9}$ to 14\% at
$10^{-6}$ Torr total pressure. Directly connected to the UHV
chamber is a small and compact high pressure cell which makes it
possible to dose molecular oxygen at pressures up to one
atmosphere. After oxygen exposure the sample was characterized by
STM and, in some cases, low-energy electron diffraction (LEED).
The transfer between the high pressure cell and the UHV chamber
could be accomplished within less than 15 min of oxygen exposure.
Most of the STM experiments were performed at room temperature,
although some of the STM images were recorded at liquid nitrogen
temperature, which sometimes resulted in higher spatial
resolution.

For the XPS measurements the surfaces were prepared and initially
characterized in the STM chamber and then brought to the SX-700
beamline\cite{Uggerhoej1995} of ASTRID in a vacuum suitcase with a
base pressure better than $1 \times 10^{-9}$ Torr. The pressure
during transfer from the vacuum chambers to the suitcase, which
could be accomplished within a minute, rose to in between $1
\times 10^{-8}$ and $1 \times 10^{-7}$ Torr. The base pressure of
the UHV chamber of the SX-700 beamline is $1 \times 10^{-10}$
Torr. After the XPS measurement the sample was transferred back to
the STM chamber and the status of the sample reinvestigated by STM.
The control experiments ensured that the examined structures were still present
and clearly visible in the STM, although some minor deterioration of
the surface might have occurred.

The Ag(111) surface was cleaned by repeated sputter/annealing
cycles. It was then oxidized by exposing it to either atomic
oxygen at partial atomic oxygen pressures of $1 \times 10^{-9}$ to
$1 \times 10^{-6}$ Torr or molecular oxygen at pressures between
0.5 and 10~Torr. During oxygen exposure the  Ag(111) sample was
held at temperatures in between 420 and 600~K for an exposure time
of 10 to 50~min. Following this recipe, the structure of the
oxidized surface varied with oxygen pressure, sample temperature,
and exposure time. As discussed below, the prepared surface
typically exhibited co-existing domains of different surface
phases. However, it was possible to prepare single phase surfaces
for some of the structures presented below by carefully tuning the
preparation parameters.

Bare \textit{et al.},\cite{Bare1995} have proposed an alternative
preparation procedure, in which the surface is exposed to NO$_2$
at pressures between $5 \times 10^{-8}$ to $5 \times
10^{-6}$~Torr, while the sample is kept at the same temperatures
as described above. This method has also been used in a variety of
subsequent
studies.\cite{Carlisle2000a,Carlisle2000b,Scheer2002,Huang2002,Huang2002a,Bocquet2003,Huang2003,Webb2004,Alemozafar2005,Reichelt2007a,Klust2007,Zhou2008a}
Here we do not present any results obtained on samples prepared by
this NO$_2$ method, although we did carry out a limited number of
such studies. Consistent with Carlisle \textit{et
al.},\cite{Carlisle2000b} we were able to produce the atomic
oxygen, $p(4 \times 5 \sqrt{3})$rect-O, and $p(4 \times 4)$-O
phases in these studies, which indicates that the use of NO$_2$
instead of atomic or molecular oxygen does not significantly
change the picture drawn up here.

All the density functional theory (DFT) calculations reported here
were carried out with the Perdew-Burke-Ernzerhof generalized
gradient approximation\cite{Perdew1996} in periodic supercells
within the plane-wave pseudopotential formalism as implemented in
the CASTEP code.\cite{Segall2002} As discussed below, a variety of
unit cells were used, all consisting of five layer Ag(111) slabs
and with fine Monkhorst-Pack \textit{\textbf{k}}-point meshes
equivalent to at least 24$\times$24$\times$1 per (1$\times$1)
surface unit cell. During structure optimizations the top layer of
Ag atoms as well as the oxide overlayer atoms were allowed to
relax, whilst the bottom four layers of Ag were held fixed. From
the DFT results simulated STM images were obtained using the
simple Tersoff-Hamann approximation.\cite{Tersoff1985} A ``tip''
height of 2.5 \AA\ above the highest atom in each overlayer for
occupied states within 2 eV of the Fermi level was used.

\section{Results and discussion}

\subsection{Overview of the observed surface structures}

Figure~\ref{FigAllStructures} displays STM images of all the
phases which we have observed experimentally. The symmetry
assignments ($c(3 \times 5 \sqrt{3})$rect, $p(4 \times 5
\sqrt{3})$rect, and $c(4 \times 8)$ for the hitherto non-assigned
phases; see below for a more detailed description) were derived on
the basis of the STM results. The middle parts of the panels
represent the fast Fourier transform (FFT) of the STM images,
while the right-hand parts reproduce LEED simulations obtained
from the symmetry assignments provided on the basis of the STM
observations. These LEED results are in excellent agreement with
the FFTs, which lends further credibility to the STM-based
symmetry assignments for the oxygen-induced phases.

At the lowest oxygen coverages an apparently disordered
arrangement of depressions is observed in the constant current STM
images (Fig.~\ref{FigAllStructures}(a)). The depressions are
interpreted as atomic oxygen adsorbates, consistent with previous
studies\cite{Carlisle2000b} and the well-known fact that oxygen
depletes the local density of states at the Fermi level, leading
to a depression-like appearance of atomic oxygen on metal
surfaces.\cite{Besenbacher1996} Domains of this disordered
structure are also frequently observed to co-exist with domains of
the structures shown in Figs.~\ref{FigAllStructures}(b) to (d).

\begin{table}
    \caption{\label{TableConditions}Summary of the structures, expected oxygen coverage, and preparation conditions
    for oxygen on Ag(111). For each preparation the first unequivocal
    identification (albeit not necessarily with the correct symmetry assignment and geometry)
    in literature is given. The asterisk indicates that the structure has been observed in the
    present study.}
    \begin{center}
    \begin{ruledtabular}
    \begin{tabular}[t]{ccccc}
        Structure & Oxygen   & Atomic & Molecular & NO$_2$ \\
                  & coverage & oxygen & oxygen    &        \\
        \hline
        Atomic oxygen & $\leq$ 0.05 ML\footnotemark[1] & $\times$\footnotemark[2]$^{,\ast}$
                      & $\times$\footnotemark[3]$^{,\ast}$
                      & $\times$\footnotemark[1]$^{,\ast}$ \\
        $p(\sqrt{3} \times \sqrt{3})$R30${^\circ}$ & 0.33 ML\footnotemark[4] &
                                               & $\times$\footnotemark[5]
                                               &                           \\
        $p(4 \times 5 \sqrt{3})$rect & 0.375 ML & $\times$\footnotemark[6]$^{,\ast}$
                                    & $\times$\footnotemark[3]$^{,\ast}$
                                    & $\times$\footnotemark[1]$^{,\ast}$ \\
        $c(3 \times 5 \sqrt{3})$rect & 0.4 ML & $\times$\footnotemark[6]$^{,\ast}$
                                     & $\times$\footnotemark[2]$^{,\ast}$
                                     & \\
        $p(4 \times 4)$ & 0.375 ML & $\times$\footnotemark[6]$^{,\ast}$
                        & $\times$\footnotemark[7]$^{,\ast}$
                        & $\times$\footnotemark[8]$^{,\ast}$ \\
        $p(7 \times 7)$ & 0.37 ML\footnotemark[9] &
                        & $\times$\footnotemark[10]
                        & \\
        $c(4 \times 8)$ & 0.25 ML\footnotemark[11] & $\times$\footnotemark[2]$^{,\ast}$
                        &
                        & \\
        Stripe phase    & ? &  $\times$\footnotemark[2]$^{,\ast}$
                          & $\times$\footnotemark[2]$^{,\ast}$
                          & \\
    \end{tabular}
    \end{ruledtabular}
    \footnotetext[1]{Reference \onlinecite{Carlisle2000b}.}
    \footnotetext[2]{Present work.}
    \footnotetext[3]{Reference \onlinecite{Reichelt2007}.}
    \footnotetext[4]{Based on the model in Ref.
    \onlinecite{Bao1993}.}
    \footnotetext[5]{A $p(\sqrt{3} \times \sqrt{3})$R30${^\circ}$ overlayer
    was identified by M{\"{u}}ller\cite{Mueller1966} on epitaxially grown
    Ag(111) films for relatively small oxygen exposures (100 L).
    Later results on a Ag(111) single crystal, which show no formation
    of overlayers after oxygen exposure up to 3000 L under
    otherwise similar conditions, contradict these initial
    experiments.\cite{Engelhardt1976} An overlayer with a local
    $p(\sqrt{3} \times \sqrt{3})$R30${^\circ}$
    symmetry was once again reported by Bao \textit{et al.} who used
    STM to study Ag samples exposed to pressures around one
    atmosphere for long times on the order of days\cite{Bao1993,Bao1996}
    (see also Ref.~\onlinecite{Schubert1995}). Overall, the observed phase
    was argued to be of higher-order commensurability.}
    \footnotetext[6]{Reference \onlinecite{Schnadt2006}.}
    \footnotetext[7]{Reference \onlinecite{Rovida1972}.}
    \footnotetext[8]{Reference \onlinecite{Bare1995}.}
    \footnotetext[9]{Based on the model in Ref. \onlinecite{Reicho2007}.}
    \footnotetext[10]{Reference \onlinecite{Reicho2007}.}
    \footnotetext[11]{Based on the tentative model in Section~\ref{Secc4x8stripe}.}
    \end{center}
\end{table}

\begin{table*}
    \caption{\label{TableSinglePhases}Summary of the preparation conditions, for which surfaces were obtained
    characterized by the predominance of a single phase.}
    \begin{center}
    \begin{ruledtabular}
    \begin{tabular}[t]{ccccc}
        Structure & Oxidizing agent & Pressure & Sample temperature & Exposure time \\
        \hline
        $p(4 \times 5 \sqrt{3})$rect                 & Atomic O & $5 \times 10^{-9}$ Torr & 500 K       & 10 min  \\
        $p(4 \times 5 \sqrt{3})$rect + bare Ag(111)  & Atomic O & $1 \times 10^{-8}$ Torr & 450 - 500 K & 5 - 15 min  \\
        $c(3 \times 5 \sqrt{3})$rect\footnotemark[1] & Atomic O & $1 \times 10^{-7}$ Torr & 500 K       & 10 - 20 min  \\
        $p(4 \times 4)$                              & O$_2$    & 10 Torr                 & 500 K       & 10 min \\
        Stripes                                      & Atomic O & $1 \times 10^{-6}$ Torr & 500 K       & 10 min \\
    \end{tabular}
    \end{ruledtabular}
    \footnotetext[1]{Contained "defects" of the $p(4 \times 5 \sqrt{3})$rect phase (cf. Figure~\ref{FigAllStructures}(c)).}
    \end{center}
\end{table*}

The structure in Fig.~\ref{FigAllStructures}(b) was previously
observed by Carlisle \textit{et al.}\cite{Carlisle2000b} and was
assigned to the decomposition of the $p(4 \times 4)$ structure in
panel (d). The structure, which we briefly mentioned in a previous
report,\cite{Schnadt2006} has a $p(4 \times 5 \sqrt{3})$rect unit
cell in the convenient terminology for rectangular symmetries on
hexagonal surfaces introduced by Biberian and van
Hove.\cite{Biberian1984} In matrix notation the unit cell (the
conventional and primitive unit cells are identical) is described
by
\begin{displaymath}
    \left(
    \begin{array}{rr}
        4 & 0 \\
        5 & 10
    \end{array}
    \right).
\end{displaymath}

The structure in Fig.~\ref{FigAllStructures}(c) also possesses a
rectangular symmetry with a conventional unit cell of $c(3 \times
5 \sqrt{3})$rect symmetry. The brighter lines from left to right
are units of the $p(4 \times 5 \sqrt{3})$rect structure. We have
observed that domains of the $c(3 \times 5 \sqrt{3})$rect
structure always contain at least small fractions of defects of
$p(4 \times 5 \sqrt{3})$rect symmetry, which in room temperature
experiments sometimes have been observed to move along the [1$\bar{1}$0]
direction. The primitive unit cell of the $c(3 \times 5
\sqrt{3})$rect phase is given by\cite{PreviousNotation}
\begin{displaymath}
    \left(
    \begin{array}{rr}
        3 & 0 \\
        4 & 5
    \end{array}
    \right).
\end{displaymath}

Panel (d) of Fig.~\ref{FigAllStructures} shows the well-known $p(4
\times 4)$ structure, which has been the subject of surface
science investigations since the early 1970s, whereas the
structures in panels (e) and (f) are reported here for the first time.
The first of these has a $c(4 \times 8)$ symmetry, while the
appearance of the second structure is characterized by stripes of
variable thickness along the \{1$\bar{1}$0\}\mbox{ } surface directions. In
matrix notation the primitive unit cell of the $c(4 \times 8)$
structure is given by
\begin{displaymath}
    \left(
    \begin{array}{rr}
        3 & 1 \\
        1 & 3
    \end{array}
    \right).
\end{displaymath}

Table~\ref{TableConditions} summarizes the conditions, which here and
in other studies have been used to prepare the different surface
structures. Both the atomic and molecular oxygen recipes are very
versatile in terms of the variety of structures which can be
produced. The NO$_2$ recipe has so far primarily been used to
prepare oxidized Ag(111) surfaces covered entirely by
the $p(4
\times 4)$
structure.\cite{Bare1995,Carlisle2000a,Carlisle2000b,Schmid2006,Reichelt2007a}
The present results show that also the $p(4 \times 5
\sqrt{3})$rect structure can be produced by exposing the Ag(111) crystal to NO$_2$. Due to the
similarity of the $p(4 \times 4)$, $p(4 \times 5 \sqrt{3})$rect,
and $c(3 \times 5 \sqrt{3})$rect structures (see below) it can be
expected that it also is possible to form the latter using NO$_2$,
while it is more difficult to draw conclusions along these lines
regarding the remaining structures.

For most preparation conditions a variety of co-existing surface
structures appear. Only for the preparation conditions summarized
in Table~\ref{TableSinglePhases} we were able to obtain
single-phase or close-to single-phase surfaces. The otherwise
observed co-existence of different surfaces structures is
illustrated in Fig.~\ref{FigStructuresAtthesametime}, which
displays an STM image recorded after exposing the surface to $5
\times 10^{-8}$~Torr atomic oxygen at 500~K for 40~min. In this
STM image the $p(4 \times 5 \sqrt{3})$rect, $c(3 \times 5
\sqrt{3})$rect, $p(4 \times 4)$, $c(4 \times 8)$, and stripe
structures are found to co-exist within a 250~{\AA} $\times$
250~{\AA} area of the surface. It is observed that the $p(4 \times
5 \sqrt{3})$rect, $c(3 \times 5 \sqrt{3})$rect, and $p(4 \times
4)$ structures have very similar STM appearances with respect to
their height profiles (brightness), whereas the STM height profile
for the the $c(4 \times 8)$ structure is much lower. As will be
seen and discussed in further detail below this reflects that the
atomic geometries of the $p(4 \times 5 \sqrt{3})$rect, $c(3 \times
5 \sqrt{3})$rect, and $p(4 \times 4)$ structures are indeed very
similar.

\begin{figure*}
    \begin{center}
        \includegraphics[width=17.8cm]{FigG.png}
    \end{center}
    \caption{\label{FigBuildingBlockStructures}(Color online)
    (a), (c), (e) STM images and (b), (d), (f) experimentally derived structures of the Ag$_6$/Ag$_{10}$ building block structures.
    The structural models for the $p(4 \times 4)$ and $c(3 \times 5 \sqrt{3})$rect phases
    in panels (b) and (d) also contain the in
    Ref.~\protect\onlinecite{Schnadt2006} theoretically derived
    positions of the oxygen atoms.
    The red/dark gray spheres represent the oxygen, (medium)
    gray the overlayer silver, and the open spheres the
    Ag(111) substrate atoms. The blue lines are guides for the eye indicating the
    Ag$_6$ and Ag$_{10}$ building blocks.
    (a) $35 \mbox{ \AA} \times 35 \mbox{ \AA}$, -0.42~nA, -21.7~mV. $p(4 \times 4)$.
    (b) $35 \mbox{ \AA} \times 35 \mbox{ \AA}$, -0.40~nA, -34.2~mV. $c(3 \times 5 \sqrt{3})$rect.
    (c) $35 \mbox{ \AA} \times 35 \mbox{ \AA}$, -0.38~nA, -17.4~mV. $(4 \times 5 \sqrt{3})$rect.
    The inset in panel (c) shows a highly resolved Ag$_6$ dimer with all 12 Ag atoms visible (-0.42~nA, -21.7~mV).
    The light (dark) arrows mark the voids characteristic for the $p(4 \times 4)$ ($c(3 \times 5 \sqrt{3})$rect-like)
    phase. Both types of voids are found in the
    $p(4 \times 5 \sqrt{3})$rect structure.}
\end{figure*}

\begin{figure}
    \begin{center}
        \includegraphics[width=8.6cm]{FigH.png}
    \end{center}
    \caption{\label{FigTheoryBBStructures}(Color online) (a) to (c) Final geometrical models of the
    building block structures $p(4 \times 4)$, $c(3 \times 5 \sqrt{3})$rect, and
    $p(4 \times 5 \sqrt{3})$rect. (d) to (f) Tersoff-Hamann simulations of the structures in (a) to (c).
    The dark (light) gray spheres represent Ag atoms in the overlayer (substrate) and the smaller red spheres O atoms.
    The blue lines are guides for the eye indicating the
    Ag$_6$ and Ag$_{10}$ building blocks.}
\end{figure}

\begin{figure}
    \begin{center}
        \includegraphics[width=8.6cm]{FigI.png}
    \end{center}
    \caption{\label{FigAg6TrianglesFccAndHcp}(Color online) Building block structures imaged
    with special tip states. (a) and (b) Imaging conditions which let the hcp and fcc Ag$_6$ triangles
    appear
    differently. (a) $100 \times 100$~{\AA}$^2$, 0.54~nA, 9.8~mV. (b) $100 \times 100$~{\AA}$^2$,
    0.85~nA, 55~mV. (c) and (d) Images of the same area in (c) normal imaging mode and (d)
    oxygen imaging mode. $47 \times 37$~{\AA}$^2$. (c) 1.48~nA, 55~mV. (d) 1.49~nA, 55~mV.}
\end{figure}

\subsection{Ag$_6$/Ag$_{10}$ building block structures\label{SecBuildingBlockStructures}}

The STM images of the $p(4 \times 5 \sqrt{3})$rect, $c(3 \times 5
\sqrt{3})$rect, and $p(4 \times 4)$ phases in
Figs.~\ref{FigAllStructures}(b-d) are characterized by ordered
arrays of unresolved bright features. Higher resolution images are
depicted in Figs.~\ref{FigBuildingBlockStructures}(a), (c), and
(e). From these it is seen that the bright features contain atomic
sub-structure. In the case of the $c(3 \times 5 \sqrt{3})$rect
phase a particular tip state even allowed all Ag atoms in the
overlayer to be resolved (inset of panel (c)). For the $c(3 \times
5 \sqrt{3})$rect and $p(4 \times 4)$ phases the arrangement of the
silver and oxygen atoms has already been discussed in detail in
Ref.~\onlinecite{Schnadt2006}. In brief, both structures are built
from close-to identical elements of Ag$_6$ triangles with the Ag
atoms of a single Ag$_6$ all located either in fcc or hcp sites.
Each primitive unit cell contains one fcc triangle and one hcp
triangle. The site difference implies a slight asymmetry between
the two triangles of a dimer, which, at least for the $p(4 \times
4)$ phase, is by DFT shown to be further enhanced by small
rotations in converse directions for the fcc and hcp
triangles\cite{Schmid2006,Reichelt2007a} [cf.
Fig.\ref{FigTheoryBBStructures}]. Indeed, for certain tip states
fcc triangles and hcp triangles are imaged differently in STM, as
is illustrated in Figure~\ref{FigAg6TrianglesFccAndHcp}(a-b). On
the basis of the STM measurements we thus propose the structural
models depicted in Figures~\ref{FigBuildingBlockStructures}(b) and
(d). It is difficult to provide a detailed assignment for the
location of the oxygen atoms, which normally are not visible in
STM due to the depletion of the local density of states at the
Fermi level typical for oxygen (see, however, below for an example
in which some oxygen density is visible). For the $c(3 \times 5
\sqrt{3})$rect and $p(4 \times 4)$ phases the oxygen atom
positions were derived by evaluating a large variety of different
structures in DFT calculations.\cite{Schnadt2006,Schmid2006} The
positions of the oxygen atoms in the thus derived structural
models have been included in the models shown in
Fig.~\ref{FigBuildingBlockStructures}(b) and (d), which were
suggested on the basis of the experimental STM results.
Figures~\ref{FigTheoryBBStructures}(a) and (b) show the structural
models emerging from the calculations. Particulary noteworthy are
the converse rotations of the Ag$_6$ triangles in the $p(4 \times
4)$ structure already mentioned above. The STM simulations, which
correspond to the calculated structures, are shown in panels (d)
and (e) and are found to be in good agreement with the
experimental STM results in
Figure~\ref{FigBuildingBlockStructures}(a) and (c). The DFT
adsorption energies (eV/O) are provided in
Table~\ref{TableStoichiometryDensity}. They are seen to be the
same to within the uncertainty of the DFT results.

The arrangement of the Ag atoms in the $p(4 \times 5
\sqrt{3})$rect structure in
Figure~\ref{FigBuildingBlockStructures}(e) can be understood from
an extension of the Ag$_6$ building block model for the $c(3
\times 5 \sqrt{3})$rect and $p(4 \times 4)$ phases. The larger
triangles constitute an additional Ag$_{10}$ building block, which
typically appears in dimer form, again with half of the silver
atoms in fcc and the other half in hcp sites. In the pure $p(4
\times 5 \sqrt{3})$rect phase the Ag$_6$ and Ag$_{10}$ dimers are
then arranged as depicted in
Fig.~\ref{FigBuildingBlockStructures}(f). It is noteworthy that
the $p(4 \times 5 \sqrt{3})$rect structure appears as a
mixture of the $p(4 \times 4)$ and $c(3 \times 5
\sqrt{3})$rect overlayers, with the voids between four Ag$_{10}$
and two Ag$_6$ triangles being $c(3 \times 5 \sqrt{3})$rect-like (dark
arrow in Fig.~\ref{FigBuildingBlockStructures}(f)) and those
between four Ag$_6$ and two Ag$_{10}$ being $p(4\times4)$-like
(light arrow).

As in the cases of the $p(4 \times 4)$ and $c(3
\times 5 \sqrt{3})$rect phases discussed above the oxygen atoms incorporated into
the $p(4 \times 5 \sqrt{3})$rect structure are normally not
visible in STM. Again, we carried out DFT calculations for a large number
of different arrangements of both oxygen and silver atoms to find
the most stable arrangement of surface atoms. This search also
included geometries which deviated from the experimentally derived
Ag$_{6}$/Ag$_{10}$ building block structure. Among the
investigated structures, the one displayed in
Figure~\ref{FigBuildingBlockStructures}(f) was found to be the
most stable one. The model is indeed based on the two different
building blocks suggested above on the basis of the STM results. The
oxygen atoms reside in the same sites as for the $p(4 \times 4)$
and $c(3 \times 5 \sqrt{3})$rect phases, a finding which lends
further credibility to the proposed model. The DFT oxygen
adsorption energy (eV/O) (cf.
Table~\ref{TableStoichiometryDensity}) is seen to be the same, to
within the DFT uncertainty, as that of the other two Ag$_6$
building block structures.

Fig.~\ref{FigTheoryBBStructures} contains the final structural
models for all three Ag$_6$/Ag$_{10}$ phases in panels (a)-(c).
The Figure also shows that the Tersoff-Hamann (TH) STM simulations
are in good agreement with the STM results. In particular, the TH
STM simulations reproduce the experimentally observed high
apparent height of the silver atoms in the centers of the
Ag$_6$/Ag$_{10}$ triangles, which are characterized by a high
silver coordination. The lower-Ag-coordinated edge atoms and even
more so the corner atoms of the Ag triangles have a much lower
local density of states intensity and therefore appear lower than
the triangle center atoms. We also note that by slightly modifying
the simulated tip apex height and bias conditions  we are not only
able to reproduce the experimentally observed triangular
appearance of the building blocks shown in
Figs.~\ref{FigTheoryBBStructures}(d-f), but also the more
spherical appearance of the bright features in
Fig.~\ref{FigAllStructures}(b-d).

The above derived models can explain the frequently observed
alignment of defects of the $p(4 \times 5 \sqrt{3})$rect type
embedded in a $c(3 \times 5 \sqrt{3})$rect domain. An example is
shown in Fig.~\ref{FigAllStructures}(c). A comparison of the
structural models for the $p(4 \times 5 \sqrt{3})$rect and $c(3
\times 5 \sqrt{3})$rect phases shows that the local geometry of
the Ag$_6$ building blocks is very similar in both phases, and, in
particular, the distance between the Ag$_6$ triangles along the
[11$\bar{2}$] surface direction is the same for both structures.
Thus, an aligned line of Ag$_{10}$ triangles, which is the second
building block of the $p(4 \times 5 \sqrt{3})$rect structure, fits
very well into a domain of the $c(3 \times 5 \sqrt{3})$rect phase.
If now, in a thought experiment, a single Ag$_{10}$ triangle is
moved from an aligned line to an isolated position, the structural
mismatch between the Ag$_{10}$ and Ag$_6$ triangles will either
lead to a reduction of the number of oxygen atoms accommodated in
the structure or, alternatively, to less favorable Ag-O bond
geometries. Either possibility will be accompanied by an energetic
cost, which, deeming from the experimental results, is not fully
compensated by the gain in configurational entropy.

In Table~\ref{TableStoichiometryDensity} we compare the
stoichiometries and top layer silver and oxygen atom densities of
the three Ag$_6$/Ag$_{10}$ building block structures. Both the
$p(4 \times 4)$ and $c(3 \times 5 \sqrt{3})$rect phases have a
Ag$_2$O stoichiometry, but the $c(3 \times 5 \sqrt{3})$rect has
slightly higher Ag and O atom densities than the $p(4 \times 4)$.
Judging from the stoichiometry the $p(4 \times 5 \sqrt{3})$rect
phase seems to be more oxygen-deficient; however, the absolute
oxygen density is the same for the $p(4 \times 4)$ and $p(4 \times
5 \sqrt{3})$rect structures, while the Ag density is increased for
the $p(4 \times 5 \sqrt{3})$rect and reaches the same level as in
the $c(3 \times 5 \sqrt{3})$rect. It is thus not correct to view
the $p(4 \times 5 \sqrt{3})$rect phase as a decomposition phase of
the $p(4 \times 4)$, as suggested by Carlisle \textit{et
al.}\cite{Carlisle2000b} While this characterization applies to
the $p(4 \times 5 \sqrt{3})$rect in relationship to the $c(3
\times 5 \sqrt{3})$rect phase, the $p(4 \times 5 \sqrt{3})$rect
structures rather corresponds to a Ag-rich reconstruction of the
$p(4 \times 4)$ structure.

From the similar geometrical arrangements of the silver and oxygen
atoms in the three Ag$_6$/Ag$_{10}$ building block structures as
revealed in the STM images it seems likely that the electronic and
chemical characteristics of these phases should be very similar.
This is corroborated by the O~1s x-ray photoelectron spectra
depicted in Fig.~\ref{FigO1s}. Within the measuremental
uncertainty of 0.1~eV the O~1s binding energies are the same for
all three structures (528.5~eV) and they agree quite well with
previous measurements on the $p(4 \times 4)$
phase\cite{Campbell1985} (528.1~eV), although they are about
0.4~eV higher in energy (they are more similar to the recent
measurement by Reichelt \textit{et al.},\cite{Reichelt2007} who
report a binding energy of 528.3~eV). However, the O~1s peak
observed for the $p(4 \times 4)$ phase prepared from atomic oxygen
is located at 528.7~eV, and this upwards shift in energy we cannot
explain at present. It is further noted that all spectra contain a
high energy component at energies between 530.1 and 530.5~eV. This
peak is particularly pronounced for the $p(4 \times 4)$ phase
prepared using molecular oxygen, i.e., at an elevated pressure.
Previously, O~1s features at high binding energy have been
assigned to both
carbonates\cite{Rehren1991,Bukhtiyarov1999,Reichelt2007}, a
reactive oxygen species or chemisorbed
oxygen\cite{Bare1995,Bukhtiyarov1999,Reichelt2007}, and -- for the
Ag(110), Ag(210), and Ag(111) surfaces as well as polycrystalline
silver foil -- to subsurface or bulk-dissolved
oxygen.\cite{Rehren1991,PawelaCrew1995,Bao1996,Savio2006,Klust2006}
Considering that contaminations released from the reactor walls
are common in high pressure experiments such as used in the
molecular oxygen preparation of the $p(4 \times 4)$ phase (10~Torr
in the present case) it seems most likely that a large part of the
peak is related to carbonates. On the basis of the available data
we cannot, however, provide a conclusive assignment of this high
binding energy O~1s peak. Indeed, it is fully possible that this
peak contains contributions from several different oxygen species.

Finally it is noted, that it is sometimes possible to directly
image some of the surface oxygen density in the STM images (see,
e.g., also Ref.~\onlinecite{Merte2009}). As stated above, for
normal STM tunneling conditions oxygen adsorbed on Ag is not
easily identified in the STM images, but, as demonstrated in
Figure~\ref{FigAg6TrianglesFccAndHcp}(d) for a mixed $c(3 \times
\sqrt{5})$rect/ $p(4 \times \sqrt{5})$rect surface, rare tip
states seem to enhance those oxygen atoms which are coordinated by
four top layer silver atoms. The oxygen atoms are identified from
a comparison to the STM image in normal imaging mode in
Figure~\ref{FigAg6TrianglesFccAndHcp}(c) and the structural models
in Figure~\ref{FigBuildingBlockStructures}(d-f). In this special
tip state image the silver atoms of the smaller Ag$_6$ triangles
are not imaged at all, while the centers of the Ag$_{10}$
triangles appear nearly as bright as the fourfold-coordinated
oxygen atoms. The threefold-coordinated oxygens, however, remain
invisible even for the special tip state.

\begin{table*}
    \caption{\label{TableStoichiometryDensity}Stoichiometry, top layer Ag and O atom densities, and DFT adsorption energies $E_{ads}$
    of the
    here described O/Ag overlayers. The data for the $c(4 \times 8)$ structure are tentative, since the geometry model presented
    in the text so far has not been confirmed by further experimental and/or theoretical methods. $E_{ads}$ is defined
    as in Reference~\onlinecite{Michaelides2005}.}
    \begin{center}
    \begin{ruledtabular}
    \begin{tabular}[t]{ccccc}
                                     & Stoichiometry     & Ag density                 & O density                   & $E_{ads}$ (eV/O) \\
        \hline
        $p(4 \times 4)$              & Ag$_2$O           & 0.104 \AA$^{-2}$ (0.75 ML) & 0.052 \AA$^{-2}$ (0.375 ML) & 0.46 \\
        $c(3 \times 5 \sqrt{3})$rect & Ag$_2$O           & 0.111 \AA$^{-2}$ (0.80 ML) & 0.055 \AA$^{-2}$ (0.40 ML)  & 0.39 \\
        $p(4 \times 5 \sqrt{3})$rect & Ag$_2$O$_{0.94}$  & 0.111 \AA$^{-2}$ (0.80 ML) & 0.052 \AA$^{-2}$ (0.375 ML) & 0.42 \\
        $c(4 \times 8)$              & Ag$_2$O$_{1.6}$   & 0.043 \AA$^{-2}$ (0.31 ML) & 0.035 \AA$^{-2}$ (0.25 ML)  &  \\
    \end{tabular}
    \end{ruledtabular}
    \end{center}
\end{table*}

\begin{figure}
    \begin{center}
        \includegraphics[width=8.6cm]{FigJ.png}
    \end{center}
    \caption{\label{FigO1s}O~1s x-ray photoelectron spectra of the indicated preparations. The specifications "molecular oxygen" and "atomic oxygen"
    refer to which preparation method was used (cf. Table~\protect\ref{TableConditions}).}
\end{figure}

\begin{figure*}
    \begin{center}
        \includegraphics[width=17.8cm]{FigK.png}
    \end{center}
    \caption{\label{Figp4x4Fillings}(Color online) The $p(4 \times 4)$ phase as a host--guest architecture.
    The images were recorded on a sample prepared using atomic oxygen.
    (a) $90 \mbox{ \AA} \times 90 \mbox{ \AA}$, -0.35~nA, -15.9~mV.
    (b) $90 \mbox{ \AA} \times 90 \mbox{ \AA}$, -0.43~nA, -1030.3~mV.
    (c) $90 \mbox{ \AA} \times 90 \mbox{ \AA}$, -0.44~nA, -2024.2~mV.
    (a) and (c) were recorded at the same time (forward and backwards movements of the tip). (b) was recorded immediately
    prior to (a) and (c), and it was also accompanied by a simultaneous recording at low bias (-33~V). The latter image
    showed the same characteristics as the image in panel (a). The sample positions differ slightly between (a)/(c) and
    (b) due to thermal drift; the spatial relationship can, however, be inferred from the pattern of the hexagon
    fillings. Identical absolute positions have been marked by a cross.
}
\end{figure*}

\subsection{\label{SecHostGuest}The $p(4 \times 4)$ phase as a host-guest structure}

In Figure~\ref{Figp4x4Fillings} STM images of the $p(4 \times 4)$
phase are shown, which were recorded at varying bias voltages. As
shown in panel (a), at the lowest biases of approximately
$-15$~meV the STM image exhibits the typical honeycomb appearance
with the structural voids displayed as depressions (cf. the atomic
model in Fig.~\ref{FigBuildingBlockStructures}(b)). When the bias
is increased to about $-1000$~mV a fraction of the voids appears
filled (panel (b)). At the highest biases used, around $-2000$~mV,
the fillings already visible at the intermediate bias are now the
brightest features in the STM image, while other voids display
fillings with a smaller apparent height in the image [panel (c)].
Only a very minor fraction of voids seem to be unfilled.

We frequently observed the presence of fillings in the voids of
the $p(4 \times 4)$ structure; indeed, it is likely that they are
an inherent feature of the $p(4 \times 4)$ phase. The fillings are
most easily imaged at elevated biases of both tunneling
polarities. At low biases they normally remain unobserved, and,
thus, their absence in STM images recorded at low biases does not
imply their absence from the surface. The fillings occur in
preparations from both atomic and molecular oxygen, but we cannot
clarify here whether they exist for preparations from NO$_2$, as
well. Interestingly, we have not seen any such filling for the
$p(4 \times 5 \sqrt{3})$rect phase, although it -- in contrast to
the $c(3 \times 5 \sqrt{3})$rect structure -- contains the same
kind of voids as visible from the structural models in
Fig.~\ref{FigBuildingBlockStructures}.

We suggest that the fillings are related to adsorbate species
in the voids of the $p(4 \times 4)$ phase. Since we observed such fillings
both directly after preparation of the surface by either atomic or molecular oxygen
as well as after measurements for a time span of several hours, it seems
unlikely that rest gas contamination can be made responsible for the
fillings. Nonetheless, at present we cannot specify the chemical
nature of the fillings and we can just note that the observation
of the fillings suggests an interesting host-guest character
of the $p(4 \times 4)$ phase,
which would deserve a more systematic study.

\subsection{\label{Secc4x8stripe}The $c(4 \times 8)$ and stripe structure}

Finally we would like to discuss two additional structures not
observed previously. These are the $c(4 \times 8)$ and stripe
phases of Figure~\ref{FigAllStructures}(e-f). In both cases it is
difficult to provide the exact details of the atomic scale
structures. The reasons are that for the $c(4 \times 8)$ structure
we have so far not been able to find a recipe for preparing a
single phase $c(4 \times 8)$ surface, and for the stripe structure
there exists a larger manifold of similar, co-existing structures.
The difficulty in preparing surfaces covered by a single coherent
structure have prevented the use of averaging techniques such as
LEED, which would have resulted in useful additional information.
In the following we therefore limit ourselves to primarily
describing the characteristics of the structures to the extent
they could be extracted from the STM data.

\begin{figure}
    \begin{center}
        \includegraphics[width=8.6cm]{FigL.png}
    \end{center}
    \caption{\label{Figc4x8}(Color online) STM images and geometrical
    models of the $c(4 \times 8)$ phase. The colors are as in
    Figure~\protect\ref{FigBuildingBlockStructures}, with the
    addition that the darkest gray sphere represent the Ag atoms
    which appear isolated in the STM images.
    (a) $200 \mbox{ \AA} \times 200 \mbox{ \AA}$, -0.40~nA, -185.8~mV.
    The $c(4 \times 8)$ is here found to occur simultaneously with
    the $p(4 \times 4)$ phase. The large parallelograms depict arrays of
    four conventional unit cells of the $c(4 \times 8)$ structure.
    The superposition of the parallelogram onto the $p(4 \times
    4)$ phase confirms the $(4 \times 8)$ assignment, since
    each unit cell is seen to comprise two $p(4 \times 4)$ unit
    cells.
    (b) $60 \mbox{ \AA} \times 60 \mbox{ \AA}$, -0.34~nV, -109.6~mV.
    (c) Tentative geometry of the Ag atomes in the $c(4 \times 8)$ structure.
    The parallelograms in (b) to (d) show both the
    conventional and primitive unit cells.
}
\end{figure}

Starting with the $c(4 \times 8)$ structure, its symmetry
assignment was derived from comparing its unit cell with those of
other phases appearing in the same STM image. Such a comparison is
shown in Fig.~\ref{Figc4x8}(a). The correctness of the assignment
is supported by the good agreement between the FFT of the STM
image and the LEED simulation based on the $c(4 \times 8)$ unit
cell in Figure~\ref{FigAllStructures}(e). Since oxygen typically
is invisible in the STM images it is assumed that the bright
features in the more highly resolved STM images of the $c(4 \times
8)$ structure correspond to silver atoms, cf.
Fig.~\ref{Figc4x8}(b). This assignment leads us to suggest the
tentative top layer Ag atom geometry depicted in
Fig.~\ref{Figc4x8}(c). It is the only geometry for which the
bright features in the experimental image can be brought into
coincidence with lattice sites. In this model, 20\% of the top
layer silver atoms reside in bridging sites, while the remaining
silver atoms are found in both hcp and fcc hollow sites.

The ring-formed depressions around the Ag atoms in bridging sites
indicate that the Ag atoms are surrounded by oxygen atoms. The
placement of four oxygen atoms in the dark ring leads to the
detailed geometry shown in Figure~\ref{Figc4x8}(d). In this
geometry the coordination of the silver atoms by other top layer
silver atoms is much lower than in the building block structures
described in section \ref{SecBuildingBlockStructures}. In these
structures silver atoms with a low silver coordination such as the
corner atoms of the Ag$_6$/Ag$_{10}$ triangles were imaged with a
reduced apparent height as compared to the fully
silver-coordinated silver atoms in the centers of the triangles.
As seen clearly in Figure~\ref{Figc4x8}(a), also the $c(4 \times
8)$ phase is characterized by a much smaller apparent height than
the brightest features of the $p(4 \times 4)$ phase. We take this
as an indication that the $c(4 \times 8)$ structure indeed is more
highly oxidized than any of the building block structures.

The suggested structure for the $c(4 \times 8)$ phase possesses a
Ag$_5$O$_4$ (Ag$_2$O$_{1.6}$) stoichiometry (cf.
Table~\ref{TableStoichiometryDensity}). A very similar structure
was observed previously for a Pd$_5$O$_4$ surface
oxide.\cite{Lundgren2002} A major difference between the present
and the Pd oxide case is, however, the full commensurability of
the Ag$_5$O$_4$ unit cell, while the Pd$_5$O$_4$ is incommensurate
in one surface direction. It is seen that the proposed
stoichiometry for the $c(4 \times 8)$ is more reminiscent of AgO
than Ag$_2$O and that the density of Ag and O atoms is much lower
in the $c(4 \times 8)$ as compared to the Ag$_6$/Ag$_{10}$
building block structures. This difference in stoichiometry and Ag
and O densities in comparison to the $p(4 \times 4)$, $c(3 \times
5\sqrt{3})$, and $p(4 \times 5\sqrt{3})$ phases is surprising in
view of the coexistence of these structures such as those in
Figure~\ref{FigStructuresAtthesametime}. At present we cannot
satisfactorily explain this finding.

\begin{figure}
    \begin{center}
        \includegraphics[width=8.6cm]{FigM.png}
    \end{center}
    \caption{\label{FigStripes}(Color online) STM images of the
    stripe phase.
    (a) $200 \mbox{ \AA} \times 200 \mbox{ \AA}$, -0.41~nA, -25.0~mV.
    (b) $75 \mbox{ \AA} \times 75 \mbox{ \AA}$, -0.47~nA, 58~mV.
}
\end{figure}

Now turning to the stripe phase, we would first like to again
emphasize that there exists a larger number of co-existing
structures which appear as stripes in the STM measurements.
Probably these structures have similar, but not exactly equal
atomic geometries. That the geometries indeed differ between
different domains of the stripe phase is quite obvious from a
comparison of the displayed domains in
Figs.~\ref{FigAllStructures}(f) and \ref{FigStripes}. The common
denominator of the domains is the occurrence of stripes along
\{1$\bar{1}$0\}, but the arrangement and apparent height of the
stripes are not always the same.

While it was difficult in general to obtain high resolution STM
images on extended domains of stripe character, this was sometimes
possible on smaller domains embedded in other structures.
Figure~\ref{FigStripes} provides examples of such high-resolution
STM images. We tentatively assume that the protruding features of
these images are silver atoms. Along the \{1$\bar{1}$0\}
directions the distance between these features corresponds to the
Ag(111) nearest-neighbor distance $b=2.89$~{\AA}.  In the
perpendicular direction the distances of the rows is, however, not
a multiple of $\sqrt{3} b$, as might have been expected. By
comparison to the surrounding surface covered by domains of the
$p(4 \times 4)$, $c(3 \times 5 \sqrt{3})$rect, and $p(4 \times 5
\sqrt{3})$rect phases it is estimated that the repetitive unit
rather comprises four stripes and that this unit has a width of $4
\sqrt{3} b$. In such a unit cell it is not possible to place all
the silver atoms in the same lattice site. The differing
brightness of adjacent stripes supports this conclusion.

The position of the oxygen atoms in the stripe phase cannot be
specified in any detail from an analysis of the STM results. It
seems likely that oxygen atoms reside in between the bright
stripes. The XPS data in Figure~\ref{FigO1s} indicate that the
oxygen content is considerably higher than in the $p(4 \times 4)$
and $c(3 \times 5 \sqrt{3})$rect phases. Similar to the $c(4
\times 8)$ structure and the corner atoms of the triangles in the
building block structures, the stripes are imaged at a smaller
apparent height compared to that of the adjacent domains of
primarily $c(3 \times 5 \sqrt{3})$rect symmetry in panel (a) of
Figure~\ref{FigStripes} and primarily $p(4 \times 4)$ in panel
(b). This suggests that the silver coordination of the Ag atoms of
the stripe phase is smaller than that of the Ag atoms in the
centers of the triangles of the building block structures, which
indicates a higher oxygen coordination and thus a higher oxygen
content of the stripe phase. However, at present, we refrain from
a more quantitative analysis, partly since it is unclear to what
extent the XPS data represent fully covered surfaces and partly
since the STM data cannot provide any more detailed information.
It is noted, though, that metal/oxygen structure with quite a high
oxygen content and exhibiting a stripe appearance have been
observed on other metal surfaces such as Pt(110).\cite{Helveg2007}

\section{Conclusions}

We have shown from an interplay of STM and XPS experiments and DFT
calculations that the oxidation of the Ag(111) surface may be
structurally more complex than previously anticipated. Depending
on the preparation conditions -- oxidation agent, dose, and
pressure, exposure time, and sample temperature during and after
exposure -- a large variety of different structures are found,
essentially all of which may coexist. Among these phases we have
identified a number of structures, which are formed from
Ag$_6$/Ag$_{10}$ fundamental building blocks. This group of
structures comprises both the renowned $p(4 \times 4)$ surface as
well as two phases of $c(3 \times 5 \sqrt{3})$rect and $p(4 \times
5 \sqrt{3})$rect symmetry. The $p(4 \times 5 \sqrt{3})$rect
structure had been observed previously,\cite{Carlisle2000b} but we
have devised a structural model which is in line with the
geometries for the $p(4 \times 4)$ (Refs. \onlinecite{Schnadt2006}
and \onlinecite{Schmid2006}) and $c(3 \times 5 \sqrt{3})$rect
phases (Ref. \onlinecite{Schnadt2006}) proposed recently and
discussed in more detail here. In addition, we have observed two
additional structures of $c(4 \times 8)$ and stripe character.

The present results suggest that a complex coexistence of oxide
and oxygen adsorbate overlayers with varying contents of oxygen
may form under the conditions of industrial oxidation catalysis.
This is in contrast to the prevalent perception that the $p(4
\times 4)$ phase alone represents an adequate model for the
surface under such conditions. It is conceivable that the
complexity of the phase diagram has a profound influence on the
dynamics of the catalytic process. Hence, although an increased
understanding of the structure of the oxidized silver surface is
presently being obtained as demonstrated in this and other
studies,\cite{Schnadt2006,Schmid2006,Klust2007,Reicho2007,Reichelt2007,Reichelt2007a}
we are still far from a detailed understanding of the oxidized
silver surface under reaction conditions. It appears likely that
the structure of the silver oxide surface may vary in response to
changes in the gas composition and the chemical potential of the
reaction intermediates.

\acknowledgments

The competent assistance of the staff of the mechanical workshop
of the Department of Physics and Astronomy in Aarhus is gratefully
acknowledged. JS wishes to thank the European Commission for
funding through a Marie Curie Intra-European Fellowship. AM is
grateful to the Alexander von Humboldt foundation for partial
support of this work and the European Science Foundation and EPSRC
for a European Young Investigator Award (EURYI). Some of the
calculations performed here were made possible by AM's membership
of the UK's HPC Materials Chemistry Consortium, which is funded by
EPSRC (EP/F067496).

redpajama_set_name: RedPajamaArXiv


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