Two-Dimensional Chiral Single Domain by D-Alaninol Functionalization of Cu(100)

Simona Irrera,† Giorgio Contini,* ,† Nicola Zema,† Stefano Turchini,† Jun Fujii, ‡

Simone Sanna,† and Tommaso Prosperi*,†

Istituto di Struttura della Materia, Consiglio Nazionale delle Ricerche, Via Fosso del CaValiere 100,00133 Roma, Italy, and TASC Laboratory INFM-CNR, in Area Science Park, S.S.14, Km 163.5,I-34012 Trieste, Italy

ReceiVed: May 10, 2007; In Final Form: May 24, 2007

We report the results of chemisorption in saturating conditions ofD-alaninol on Cu(100) in term of the analysisof low-energy electron diffraction and scanning tunneling microscopy data. A large two-dimensional, singledomain, ordered chiral structure of quadrangular tetrameric molecular units is formed. The four moleculesinteract differently with the surface in the two orthogonal directions.

Introduction

Anchoring chiral molecules to a solid surface is an establishedmethod exploited by analytical chemists1 for a long time inparticular to purify and resolve chromatographically racemicmixtures of different types at the analytical and preparativelevel.2 In this frame, the immobilization of the chiral moleculeon the inert substrate favors the chiral discriminations betweenthe chiral probe and the enantiomers contained in the elutingmixture. Differently, when a chiral molecule is added to aheterogeneous catalyst a new property is provided to its surfacethat now acts as a stereospecific catalyst. This was achievedfor Ni and Pt surfaces in the hydrogenation ofâ and R ketoesters, respectively,3,4 and their mechanism revised.5 Later,several amine- and amino-alcohols have been tested as chiralmodifiers of catalytic metal surfaces, and the relationshipbetween the modifier structure and the catalyst activity has beenthoroughly investigated.6 In the past decade, a considerable efforthas been devoted to studies on the deposition of chiral moleculeson achiral crystal surfaces7 and in their characterizations withseveral methods,8 while the chirality assignment of the molecularmoiety is already debated.9,10The use of such surfaces has beenrecently extended to other fields as that of adhesion andactivations of immune cells.11 Most of the results underline theformation of monolayers composed of densely packed, differentchiral domains several tens of nanometers wide, spatiallyoriented depending on the enantiomeric species deposited.12

When a racemic mixture is adsorbed at submonolayer or insaturating conditions, isolated enantiomeric clusters or phase-separated, chirally arranged domains are obtained respec-tively.13,14 In this paper, we report the results in terms of low-energy electron diffraction (LEED) patterns and scanningtunneling microscopy (STM) images of the deposition over theCu(100) surface of theR(-) enantiomer of -2-amino--propanol(D-alaninol).

The two functional groups of alaninol have distinct reactivityto copper. While it is known that copper surfaces need apreoxidation stage to react with the OH group,15 the aminomoieties are notorious to stably bind to copper through acovalent bond either in a vacuum deposition process16 or froman acetonitrile solution.17

Recent studies carried out by us on the photoelectronspectroscopy (PES) and on the circular dichroism in the angulardistribution (CDAD) of D-alaninol in the vapor phase haveassigned a distinct chiral character to both functional groups18

when the population of the prevalent conformers is taken intoaccount.19

Experimental Details

All the described experiments were carried out at roomtemperature (RT) and in ultrahigh vacuum (UHV) conditions.Two separate apparatuses were used for the deposition-LEEDand XPS experiment and for the deposition-LEED and STMexperiment.

D-Alaninol enantiomer vials were purchased from SigmaAldrich and purged of atmospheric oxygen, nitrogen, andhumidity by repeated freeze and dry cycles with liquid nitrogenover a zeolitic molecular sieve. The purified liquid was thenintroduced into the vacuum chamber through a leak valve to adispenser placed close to the copper surface.

The deposition ofD-alaninol on the single-crystal Cu(100)surface was carried out starting from a base pressure of low10-10 mbar until a constant pressure of low 10-8 mbar wasreached. The crystal surface before and after each moleculardeposition was cleaned with repeated cycles of argon-ionsputtering at 1keV and annealing at about 700 K; possiblecontamination was checked by means of X-ray photoelectronspectroscopy (XPS) detection of C 1s and O 1s core level peaks.

LEED patterns were recorded after exposures had been carriedout from low coverage up to adsorption saturations and sharpdiffraction peaks monitored at the LEED unit. A monolayer ofD-alaninol saturates the (100) surface after 15 L of exposure.20

The LEED diffraction pattern obtained for this surface, usingprimary electron energy of 37 eV, is reported in Figure 1a.

* To whom correspondence should be addressed. E-mail: (G.C.)giorgio.contini@ism.cnr.it; (T.P.) tommaso.prosperi@ism.cnr.it.

† Istituto di Struttura della Materia.‡ TASC Laboratory INFM-CNR.

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Published on Web 06/08/2007

10.1021/jp0735684 CCC: $37.00 © 2007 American Chemical Society

The sample was then transferred through a valve into theUHV-STM unit kept at a pressure of 10-10 mbar.

STM images were obtained in constant current mode (V )300 mV, I ) 0.1 nA) after deposition of one monolayer ofD-alaninol on Cu(100) at RT.

Results

The presence of a distinct diffraction pattern in Figure 1awitnesses a long range order after molecular coverage of themetal substrate. The new surface phase has a real space unitcell, represented by the matrix (4,-1|1,4), which is rotatedclockwise by 14° with respect to the [011] direction of the metalsurface. The unit cell has dimensions 10.51 Å× 10.51 Å. Thissize might well accommodate a quadrangular tetrameric mo-lecular unit as will be seen with the following STM investiga-tion.

The STM investigation was then undertaken to gain deeperinsight into the packing at the supramolecular level and to checkfor the presence of (sub) nanometer-sized defects within thefilm. As shown in Figure 2A, in a three-dimensional (3D) imageof an area of (190× 190) Å2 a submicrocrystalline singledomain, extended over a few hundreds nm2, is obtained upondeposition at RT. A closer vision to an area of (29× 29) Å2

reveals the structural composition of the single domain at thenanoscale. The geometry of the structure is defined by a squareperiodic repeat unit whose dimensions of (10.5× 10.5) Å2

match very closely those derived from the LEED diffractiondata. The unit is composed by four bright spots inserted into aquadrangular frame of black orthogonal stripes. The bright spotshave a nearly circular shape with diameters varying between3.6 and 3.0 Å. In view of their sizes, each spot can be assignedto a single alaninol molecule that constitutes with its threeneighbors a tetrameric cluster contained in the repeat unit. STMprofiles of the tetramer alonga andb directions, as reported inFigure 2C, indicate paired peaks at distance of 3.8 and 4.4 Å,respectively.

Discussion

The STM profiles are qualitatively dissimilar in that, asreported in Figure 2 insets B and C, those measured alongaindicate that an evenly distributed contour joins adjacentmolecules, whereas well-separated lobes are sampled alongdirectionb with pronounced minima between the two molecules.The observed differences in the peak to peak distances, the form,and the sizes of the bright spots on the two sides of the clusterall together suggest that the four molecules are differentlyinteracting with the surface in the two orthogonal directions.

This twofold geometric morphology of the tetrameric unit isexplained in the view of the two photoemission binding energiesfound for nitrogen 1s states at 399.4 and 397.5 eV, which areattributed to nitrogen when it is coordinated to a metal as aprimary amine (NH2)17 or as an imine (NH),21 respectively.Given that our measurements20 evidence the existence of bothelectronic structures while the oxygen 1s line width is unbroad-ened, it is likely that the scenario is characterized by a doublecoordination of nitrogen to copper given its marked basiccharacter.22 Nitrogen bindings to the copper surface would thenexpose at the surface an amphiphilic supramolecular structurecomposed of the methyl and hydroxyl groups, each one favoringin turn intermolecular hydrophobic and hydrogen-bondinginteractions.

Molecular coverage density is 3.62 molecules per nm2, similarto those gathered for alanine on Cu(001)23 and on Cu(110),8

though the molecular array here is different with only one unitpresent, intercalated by orthogonal channels a few angstromwide, which might be sites exposing extended stripes ofmolecule-free copper surface. The resulting densely reticulatedgrid, whose contour complements the supramolecular assembly,should provide further functionalities to the metal surface.

Conclusions

In conclusion, we have succeeded in designing a large two-dimensional (2D) homochiral organic-metal network formedin situ under UHV conditions by chemisorption at RT ofD-alaninol deposited on the achiral Cu(100) surface. Thehomogeneous tetrameric clusters, which constitute the repeatunit of the single domain, have dimensions that are the smallestknown so far for enantiomeric domains. We ascribe theformation of a single domain of this system to a prevailingconformer in the vapor phase of alaninol19 and to a singlebridging group of the molecule with the metal surface. Thespatial extension of this domain stimulates further studies tofully ascertain the surface structure at a higher resolution asthat which can be achieved with grazing incidence X-raydiffraction.

These type of structures might be useful as platforms fortransfer from a 2D to a 3D hierarchical self-assembling

Figure 1. (a) LEED pattern observed at 37 eV primary electron energyshowing a (4,-1|1,4) phase ofD-alaninol on Cu(100) (main crystal-lographic directions are shown and copper spots are highlighted bycircles); (b) corresponding real lattice. Grid intersections and filled blackdots represent the copper atoms array and the molecular diffractionunit respectively; arrows indicate the corresponding unit vectors.

Figure 2. STM images obtained in constant current mode (V ) 300mV, I ) 0.1 nA) of D-alaninol monolayer on Cu(100) at RT. Imagesizes in panel A are (190× 190) Å2 and in panel B are (29× 29) Å2.In panel B, the copper [011] crystallographic direction is shown bythe white arrow. Inset C shows linescan height profiles taken acrossthe lobes along directionsa andb.

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construction. Homochiral amplification can also be triggeredby the supramolecular structure through hydrogen-bonding andhydrophobic interactions while the metal nanonetwork mightbe the site for transfer and amplification of the chiral characterof the substrate.

Acknowledgment. This work has been partially supportedby the Italian National Council of Research (CNR) doctoralfellowship grants scheme awarded to S.I.

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