Regular Array of L-Tyrosine Molecules on Si ( 111 )-Au Superstructures

Studies of adsorption of organic molecules on metal-induced super-reconstructed silicon surfaces are important for future Si-based organic electronic device. We investigated the adsorption dynamics of L-tyrosine on Si-Au surface by in situ reflection high-energy electron diffraction. Si(111)5×2-Au and Si(111) √ 3× √ 3-Au coexisting surface was exposed to L-tyrosine and the intensities of diffraction spots from corresponding domains were monitored. Ltyrosine was found to adsorb onto the Si(111)5×2-Au domains preferentially, while the Si(111) √ 3× √ 3-Au domains were found to be less active. Adsorption sites were revealed by using scanning tunneling microscopy. The molecular adsorption site on the √ 3× √ 3-Au was found to be on domain boundary suggesting that the adsorption probability is small on this domain. On the other hand, the adsorbates ordering in 2.3 unit cell distance along [1̄10] rows were found to create the similar structure as the Si(111)5×2-Au surface. These results suggest that the Si(111)5×2-Au superstructure can be used for the control of the molecular adsorption geometry and ordering effectively. [DOI: 10.1380/ejssnt.2010.303]


I. INTRODUCTION
Understanding of bonding configuration and its electronic character between adsorbed organic molecule and inorganic substrate is important for organic-inorganic hetero-junction devices such as organic electronic devices and biosensors.Adsorption of molecules on solid surface depends on the geometric structure and electronic state of surface top layer atoms.
Proteins are the basic biomolecules which are the sequences of various amino acids.By fixing amino acid molecules on the well-defined surfaces, chemical and physical responses under controlled environments can be individually studied in atomic scale.Glycine, the simplest amino acid, was found to self-assemble when adsorbed onto crystalline Cu [1], Au [2], and Si surfaces [3].Mechanism of chiral recognition between a pair of chiral cysteine adsorbates on the Au(110) surface through formation of S-Au bonding and unique intermolecular interactions were revealed by scanning tunneling microscopy (STM) observation [4].L-tyrosine has a phenol group in the side chain and act as a precursor of neurotransmitter such as dopamine and adrenaline [5].Tyrosine adsorption on Cu(111) surface forming two-dimensional molecular array in electrochemical condition was also reported [6].These arrays can be used for molecular recognitions.
Surface structure of silicon can be controlled in various ways by metal atom adsorption and annealing [7].In the case of gold adsorption on Si(111) surface, many types of surface superstructures such as Si(111)5×2-Au [8][9][10][11] In this study, the difference of the adsorption dynamics of L-tyrosine on different Si(111)-Au superstructure surfaces has been investigated by in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM).Pure Si(111)5×2-Au, pure Si(111) √ 3× √ 3-Au , and their coexisting surface were exposed to L-tyrosine.The adsorption probabilities on these superstructures were investigated by monitoring the intensities of RHEED diffraction spots from corresponding superstructure domains.Adsorption sites were investigated by using STM.

II. EXPERIMENTAL
Sample preparation and all measurements were carried out under 10 −8 Pa ultra high vacuum (UHV) condition.Au/Si(111) surface superstructures were prepared by reacted deposition epitaxy (RDE) method.Clean Si(111) surface with 7×7 structure was kept at 850 K during Au deposition.Dosage of Au was carefully controlled by using in situ RHEED observation.All the RHEED measurements were done at 15 keV.Pure Si(111)5×2-Au, pure √ 3× √ 3-Au, and their coexisting surface were prepared.The Au coverages were 0.4 ML for the pure 5×2 surface, and 0.8 ML for the pure √ 3× √ 3 surface, respectively, and it was 0.7 ML for the coexisting surface.
The chemical structure of L-tyrosine is shown in Fig. 1.
L-tyrosine (Wako, 99.0 %) was dosed onto these surfaces by thermal evaporation.Since L-tyrosine decomposed over 640 K, the molecule evaporation was carried out at 450 K by using oil-bath-type evaporator where the heating oil was agitated for homogeneous temperature heating.The deposited molecule was checked by ex situ Xray photoelectron spectroscopy (XPS) as shown in Fig. 2. We found chemical shifts corresponding to each chemical bond of L-tyrosine, which means that the molecules are not decomposed, as described later.Dosage of L-tyrosine was controlled by evaporation time.We took RHEED patterns during molecular evaporation onto several surface structures and analyzed molecular adsorption process.STM images with various L-tyrosine dosages were taken systematically by preparing wedged thickness molecular films on the Si(111)5×2-Au and √ 3× √ 3-Au substrates.After the preparation of surface superstructure and deposition of L-tyrosine, we transferred the samples to STM measurement chamber through UHV transfer system [17].STM measurements were carried out by UHV-STM system (UNISOKU, USS-3000) with homemade electrochemical polished W tips.
Figure 3 shows a measured valence band photoelectron spectrum (open circle) of L-tyrosine film on Si(111)7×7 surface that was prepared by present evaporation method and a calculated density of states of L-tyrosine molecule (solid line) using DV-Xα method.Although the density of states at the vicinity of Fermi level is suppressed in the measured spectra, the calculated density of states reproduce the valence band spectrum well.This means again that the deposited L-tyrosine was not decomposed by the present method.) and that of , respectively.5×2 spot intensity decreased more rapidly than that of √ 3× √ 3 streak.This indicates that molecule preferentially adsorb onto the 5×2 superstructure.Note that the 5×2 spot intensity curve follows an exponential decay function, while that of √ 3× √ 3 domains have a shoulder structure at 50 seconds.
Adsorption coefficients for 5×2 and Figure 6 shows a series of STM images for different Ltyrosine dosages on the Si(111) 5×2-Au surface.The row structure along [ 110] direction was obtained.Bright protrusions aligned along the 5×2 row structure in Fig. 6(a) were reported to be the dangling bonds of excess Si adatoms on the 5×2-Au surface [8,9].These Si adatoms are about 0.02 ML.In the case of low dosage, molecular adsorbates appeared as blurred features (∼20 Å) much larger than the Si dangling bond bright protrusions.The number of these features increase with the dosage while that of Si adatoms decrease as shown in Figs.6(b) and (c) suggesting that the L-tyrosine molecules bond to these Si adatoms at very initial stage.Since L-tyrosine is about 10 Å in size, blurring suggests that the molecule is anchored to Si adatom with a single bond and is moving around the bond.
Then after disappearance of Si dangling bond bright protrusions at the higher coverage, different kind of bright spots begins to appear as shown in Figs.6(f)-(h).They were similar to that assigned to Si dangling bonds but more densely aligned along the direction of the 5×2 row structure.Blurred features become minority but still exist at coverage of 0.1 ML as shown in Fig. 6(h).The histograms in Fig. 7(a) show the distribution of nearest neighbor distance of the spots along 5×2 row direction.
Open squares and open circles in Fig. 7 between two neighboring bright spots corresponding to Si adatoms was mainly 4 or 6 units in length.On the other hand in the case of higher coverage, the most probable distance was 2.3 units in length.The L-tyrosine molecule align in a periodicity of "2.3" along the row, but these is no correlation between rows.This structure, which correlated (periodicity 2.3) in one direction and uncorrelated (periodicity 5) in another direction, is similar to that of Si(111)"5×2"-Au structure [23,24].

FIG. 2 :FIG. 3 :
FIG. 2: Core level photoelectron spectra of C 1s, O 1s, and N 1s.The open circles are experimental results.Blue and red lines are fitted curves of each peak and their summation.Peaks correspond to each L-tyrosine bond.

Figure 4 3 - 3 .
FIG. 5: RHEED spot intensities as a function of deposition time.Red open squares and blue open circles are experimental results corresponding to 5×2 and √ 3× √ 3. Blue solid line and red dashed line are simulated intensity changes of 5×2 and √ 3× √ 3 spots.The signs of (a)-(d) correspond to the RHEED pattern in Fig. 4.

3 -
FIG. 7: Histograms of distances between nearest two bright protrusions along the 5×2 row direction.(a) Red open squares and blue open circles are those for the L-tyrosine coverages of 0.003 ML and 0.034 ML, respectively.The inset (b) is close-up STM image at 0.034 ML molecular coverage.

Figure 7 ( 3 - 3 -
b) is a close-up of the aligned adsorbates at the dosage of 0.034 ML.Note that the bright spots were aligned not in straight but in zigzag indicating the existence of two Au adsorption sites in a 5×2 rows.STM images of L-tyrosine adsorbed √ 3× √ Au surface are shown in Fig. 8.We can see √ 3× √ 3 spots and bright domain boundaries in Fig. 8(a) [12, 25, 26].Adsorbed molecules are seen as blurred spots in Fig. 8(b).In Fig. 8(a) bright √ 3× √ 3 islands are surrounded by linked dark domain boundaries.In Fig. 8(b) bright area (which is thought as molecules) is linked and surrounds dark islands.Hence we can conclude that the molecules are adsorbed on the domain boundary.IV.CONCLUSION Adsorption rates of L-tyrosine on Si(111)5×2 and √ 3× √ Au domains were measured by in situ RHEED measurement during molecular deposition.L-tyrosine adsorbs onto 5×2 domains preferentially than that onto √ 3× √ 3 domains.Adsorption coefficient for 5×2 domains was 1.7 times larger than that of √ 3× √ 3 domains.Ordering of L-tyrosine adsorbates on 5×2 domains was observed by STM.Adsorbates aligned along 5×2 row with a period of 2.3 unit cell.These results show that Si(111)5×2-Au superstructure can align L-tyrosine molecules in regular one dimensional chain.