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Formation of homochiral glycine/Cu(111) quantum corral array realized using alanine nuclei

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2015 Jpn. J. Appl. Phys. 54 08LB05(http://iopscience.iop.org/1347-4065/54/8S2/08LB05)
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Japanese Journal of Applied Physics 54, 08LB05 (2015)
http://dx.doi.org/10.7567/JJAP.54.08LB05

Formation of homochiral glycine/Cu(111) quantum corral array realized using alanine nuclei
Miki Nakamura, Hui Huang†, Ken Kanazawa, Atsushi Taninaka, Shoji Yoshida,
Osamu Takeuchi, and Hidemi Shigekawa*
Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
Received November 18, 2014; accepted January 13, 2015; published online July 13, 2015
Glycine has enantiomeric isomers on a Cu(111) surface through the dissociation of hydrogen from the carboxyl group and forms an array ofquantum corrals of >1.3 nm diameter. Stable homo-chiral glycinate trimers are formed in the ﬁrst step, which subsequently form a network witha hexagonal arrangement. However, domains with R- or S-chirality coexist with the same probability. On the other hand, α-alanine has D- andL-chirality in nature and forms a similar quantum corral array on Cu(111) with R- and S-chirality, respectively. Here, by using α-alanine molecules asnuclei, the chirality of glycine molecules was controlled and a homochiral quantum corral array was successfully formed, which indicates thepossibility that the optical isomers can be separated through a method such as preferential crystallization.
© 2015 The Japan Society of Applied Physics

1.

Introduction

The understanding and control of self-organization is a key
factor for the development of functional devices based on avariety of molecular characteristics. Among them, chirality
plays an important role, for example, in molecular recognition, polymerization, optical functionalization and catalytic
reactions.1–4) On the other hand, with the advancement ofsurface science techniques, analyses of the moleculardynamics on solid surfaces have provided important bases
for the development of molecular technologies as well as forunderstanding the elemental interactions producing a varietyof molecular functions. Scanning tunneling microscopy(STM) is a promising technique because of its capability toanalyze the electronic structures of target materials together
with their atomic and=or molecular structures. Manyinteresting results have been obtained in studies on selfassembled monolayers (SAMs).5–21)
Recently, STM has been applied to study SAMs of glycine
and alanine molecules on Cu(110),12,13) Cu(111),14–18) andCu(001)19–21) surfaces and has shown the importance ofchirality in their self-organization mechanisms and in
producing their characteristic electronic structures. Glycine
and alanine are interesting molecules because of their speciﬁccharacteristics regarding chirality. Among the naturallyoccurring α-amino acids, glycine is the only molecule that
does not have chirality [Fig. 1(a)], although enantiomericisomers appear on a Cu surface at room temperature (RT)through the dissociation of hydrogen from the carboxyl groupowing to the freedom of the directional relationship between
the two groups in the adsorbed form [the R- and S-ﬁguresin Fig. 1(c)]. On the other hand, alanine has two isomers,
α- and β-alanine. β-alanine has a structure with an additional
methylene group (–CH2–) in its main chain compared withglycine and does not have chirality, similarly to glycine,
while α-alanine naturally has R- or S-chirality (D- and Lalanine, respectively) as shown in Figs. 1(b) and 1(d).15,16)
For the cases of glycine and β-alanine molecular structures
on a Cu(001) surface, two-dimensional electron gas (2DEG)
states with standing waves have only been observed for thep(2 × 4) arrangement consisting of both R- and S-glycine
molecules.19) The anisotropic ratios of the eﬀective masses
were 10 for glycine19) and 3.6 for β-alanine.20) In contrast, in†

Present address: Hefei University, China.

Fig. 1. (Color) Schematic structures of (a) glycine and (b) α-alanine
molecules, and their conformations on a Cu(111) surface (c, d).

the cases of glycine and α-alanine on a Cu(111) surface,glycine forms homochiral domains consisting of R- or Schirality through the formation of enantiomeric isomers,15,17)and similar quantum corrals with R- and S-chirality areformed by D- and L-alanine, respectively.16) However, theexperimental results show the observed conﬁnement energies
formed by glycine and alanine molecules to be slightly
diﬀerent, 0.3 eV for glycine17) and 0.5 eV for alanine.16)
These results indicate that the control of interactions basedon chirality and the modiﬁcation of molecules, the addition ofa methylene group to glycine in this case, play key roles in
the formation and control of the molecular and electronic
structures of SAMs. However, few analyses have been
carried out so far on the combination of these factors.
Here, as a ﬁrst step to achieving a new methodology forcontrolling the electronic structures of SAMs, we attempted
to control the chirality of quantum corrals formed by glycine
on a Cu(111) surface by adding α-alanine as nuclei using theability of chirality recognition, which was conﬁrmed in thisstudy, between glycine and alanine molecules.
2.

Experimental procedure

A clean Cu(111) surface was prepared by three cycles of Ar+
ion bombardment and annealing at 820 K. The glycine andα-alanine sources were outgassed by heating an Al2O3crucible at 330 K for 5 h. Glycine and=or α-alanine (D- or08LB05-1© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 08LB05 (2015)

M. Nakamura et al.

Fig. 3. (Color) (a) STM image (Vs = −0.5 V, It = 1.0 nA) and(b) schematic model of quantum corrals consisting of L-alanine. Yellow lines
indicate the directions used to show the S-arrangement. White images
indicated by the red arrows show the temporally averaged motions of alanine
molecules trapped in the quantum corrals.

Fig. 2. (Color) (a) Quantum corral domains with R- and L-chirality
formed by glycine molecules (Vs = −0.5 V, It = 0.5 nA). Magniﬁed imagesof quantum corrals consisting of six trimers with (b) R- and (e) L-chirality.
Schematic models of quantum corrals (c, f) and trimers (d, g), with R- and Lchirality, respectively. Green and yellow lines indicate the directions used todiscriminate between the R- and S-arrangement, respectively.

L-alanine) were adsorbed on the Cu(111) substrate dependingon the experiment, where the source and Cu sample temperatures were maintained at 370 K and RT, respectively.
After molecular deposition on the clean Cu(111) surface, thesample was annealed at 350 K for 1 h. Then, STM measurements were performed in ultrahigh vacuum (<1 × 10−8 Pa) at
77 K using a tungsten tip.
Experiments were carried out in four steps: (1) formationof quantum corrals by glycine molecules to compare theresults with those in Ref. 17, (2) formation of quantum
corrals by each of D- and L-alanine molecules to compare theresults with those in Ref. 16, (3) formation of quantum
corrals by co-adsorption of D- and L-alanine molecules, and(4) control of the chirality of glycine molecules by using Dor L-alanine molecules as a nuclei. The last two steps show
new results obtained in this work.
3.

Results and discussion

Figure 2(a) shows a typical STM image of quantum corrals
formed by the glycine=Cu(111) structure where R- and Sglycine domains exist separately, as was observed in ourprevious study.17) Figures 2(b) to 2(g) show magniﬁcations
and schematic models. For both chiralities, the array ofquantum corrals is formed by homochiral ring structures[(b, c) and (e, f)] with six glycine trimers (d, g), which consistof three of the homochiral glycinate molecules shown inFigs. 1(a) and 1(c). The green and yellow lines indicate thedirections used to discriminate between the R- and Sarrangements, respectively.
Next, as the second step, we formed quantum corrals by
the deposition of pure D- and L-alanine molecules, which
respectively resulted in the formation of quantum corrals withFig. 4. (Color) (a) Quantum corral domains formed by the co-adsorptionof D- and L-alanine molecules with R- and L-chirality, respectively(Vs = −1.0 V, It = 0.1 nA). Magniﬁed images of quantum corrals with (b) Rand (c) L-chirality and their schematic models (d, e). Green and yellow lines
indicate the directions used to discriminate between the R- and Sarrangement, respectively. White images indicated by the red arrows show
the temporally averaged motions of alanine molecules trapped in thequantum corrals, corresponding to those in Fig. 3(a).

R- and L-chirality, as expected. Figure 3 shows a typical
STM image obtained for the case of L-alanine. Quantum
corrals with S-chirality were clearly observed. In contrast to
the case of glycine, there are white images in some quantum
corrals, as indicated by red arrows, reﬂecting the temporally
averaged images of the motion of extra alanine molecules
trapped in quantum corrals.16)
Subsequently, as the third step, quantum corrals wereformed by the co-adsorption of D- and L-alanine molecules.
Figure 4(a) shows a typical STM image of the alanine=Cu(111) surface. Figures 4(b) to 4(e) show magniﬁcations of08LB05-2© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 08LB05 (2015)

M. Nakamura et al.

the two types of domains and their schematic models. As
shown in Fig. 4(a), the D- and L-alanine molecules recognizedchirality, and homo-chiral domains with D- and L-alanine
molecules were separately formed, as was observed for thecase of glycine molecules shown in Fig. 2(a). The red arrows
indicate the temporally averaged images of the movementof alanine molecules trapped in the quantum corrals, as
was pointed out for those in Fig. 3(a). Although glycine
molecules formed a racemic body because they have chirality
upon self-organization, since domains with R- and S-chirality
appear, the body is not a general racemic compound buta racemic conglomerate in which both enantiomers are
crystallized separately. On the other hand, a racemic
conglomerate is also formed when D- and L-alanine molecules
are co-adsorbed. These results indicate the possibility that aracemic compound can be changed into a racemic conglomerate by self-organization on a Cu surface.
Although the structures shown in Figs. 2(a) and 4(a)are similar, α-alanine naturally has R- or S-chirality (Dand L-alanine, respectively), in contrast to the enantiomericisomers of glycine. Namely, in the formation of a homochiraldomain by glycine molecules, the chirality may change
between the D- and L-conformations during the annealing;
however, alanine maintains its chirality after adsorption.
Therefore, the diﬀusion of molecules must be suﬃcient toform domains whose size is as large as those of glycine. On
the other hand, there are some defects in the trimers, as
shown in Figs. 4(b) and 4(c). This is considered to be due tothe fact that even single and dimer structures are stable on
the Cu(111) surface for the case of alanine, whereas only thetrimer structure is stable for the case of glycine.
Considering the experimental results obtained for glycine
and α-alanine molecules, we attempted to control the chiralityof glycine molecules by blending a small amount of D- or
L-alanine molecules to act as nuclei to form homochiralquantum corrals of glycine, the forth step described above.
Namely, if alanine and glycine molecules recognize thechirality of each other and glycine molecules change theirchirality, if necessary, to form homochiral pairs and trimers
when they make contact with alanine molecules or reach theedge of glycine molecules trapped by the nuclei of alaninemolecules, quantum corrals with the chirality of the blended
alanine molecules, thereby their domains are supposed tobe formed.
Figures 5(a) and 5(b) show the L-alanine molecules
adsorbed on a Cu(111) surface in a wide area and a highresolution image and its magniﬁcation of the area indicatedby the yellow square, respectively. As shown in Fig. 5(b), theimage has the characteristic of trimers consisting of glycine
molecules, i.e., bright spots and three regions of depressionthat are darker than the original substrate, suggesting theelectronic structure similar to that of glycine.17) In addition
to the trimer structures, however, there are some structuresconsisting of single and double alanine molecules, indicating
the fact that alanine molecules are stable even in those forms.
After conﬁrming the structure shown in Fig. 5(a), glycine
molecules were adsorbed on the surface and Fig. 5(c) shows
the quantum corrals formed after the adsorption. As indicatedby the yellow lines, all domains consist of molecular
structures with S-chirality. Since glycine molecules areexpected to have R- and S-chirality with the same probability

Fig. 5. (Color) (a) STM image of L-alanine molecules on a Cu(111)
surface and (b) a high resolution image and its magniﬁcation of the area
indicated by the yellow square (Vs = −0.5 V, It = 1.0 nA). (c) STM image ofquantum corrals formed by glycine molecules deposited onto the surface
shown in (a). (d) Magniﬁcation of the red square area in (c). Yellow lines
indicate the directions used to show the S-arrangement.

when they are adsorbed onto the Cu(111) surface, as wasobserved in Fig. 2, they are considered to change theirchirality when they make contact with L-alanine or reach theedge of a domain grown with S-chirality around the nuclei ofL-alanine, as discussed above.
These results indicate ﬁrst the ability of chirality
recognition between glycine and alanine molecules, and thepossibility that the optical isomers can be separated througha method such as preferential crystallization, in which acrystal consisting of molecules with the same chirality isgrown by introducing an optical isomer to a racemiccompound. In addition, as shown in a magniﬁed image ofthe red squared area in Fig. 5(c) [Fig. 5(d)], quantum corrals
mostly do not seem to have alanine molecules inside, whichmakes this structure more applicable compared to the casesby D- and L-alanine described above.
4.

Conclusion

As a ﬁrst step to achieving a new methodology for controlling the electronic structures of SAMs, we attempted tocontrol the chirality of quantum corrals formed by glycine ona Cu(111) surface by adding α-alanine as nuclei using theability of chirality recognition between glycine and alaninemolecules, and an array of homochiral glycine quantum
corrals was successfully formed.
Acknowledgement

Support from the Japan Society for the Promotion of Sciencein the form of Grants-in-Aid for Scientiﬁc Research is
acknowledged (22226003).

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