One dimensional organic nanostructures a novel approach based on the selective adsorption of organic molecules on silicon nanowires

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

Surface Science Letters One-dimensional organic nanostructures: A novel approach based on the selective adsorption of organic molecules on silicon nanowires Eric Salomon * , Antoine Kahn Department of Electrical Engineering, Princeton University, E-quad, Olden Street, Princeton, NJ 08544, United States article info Article history: Received 11 March 2008 Accepted for publication 16 April 2008 Available online 26 April 2008 Keywords: Chemisorption Nanopatterning Scanning tunneling microscopy Scanning tunneling spectroscopy abstract Scanning tunneling microscopy (STM) is used to investigate the formation of one-dimensional organic nanostructures by chemisorption of specific molecules on silicon nanowires. STM data demonstrate that depending on the molecular functional groups, the molecules adsorb either randomly on the substrate or preferentially on the nanowires. In the latter case, chemisorption of suitable organic molecules on the nanowires leads to a well-defined one-dimensional aggregation and changes the metallic character of the nanowires to a semi-conducting one. Ó 2008 Elsevier B.V. All rights reserved. One-dimensional (1D) metal, semiconductor and insulator structures have attracted a great deal of interest from the scientific community because of their potential for nanotechnology and the opportunity they provide to understand the fundamentals of the physics of low-dimensional systems. Fabricating these structures with controlled pattern and dimensions remains experimentally challenging for inorganic materials, and even more so for organic ones. One method for growing 1D organic structures consists of adsorbing organic molecules on a template surface presenting a well-defined 1D-pattern. In that regard, it has been recently dem- onstrated that 16 Å wide silicon nanowires (NWs), heretofore re- ferred to as SiNWs, can be grown by adsorbing silicon atoms onto a single crystal of silver oriented with a (110) surface (Ag(110)) [1,2]. This system has been thoroughly studied by means of scanning tunneling microscopy (STM), low energy elec- tron diffraction and photoelectron spectroscopies (PESs) [1–3]. These SiNWs were shown to form massively parallel assemblies with a density of states consistent with quantized states dispersing along the NWs. This system has also been theoretically investi- gated by He, who proposed an arrangement of the Si atoms on the surface [4]. In the present work, we use this template of SiNWs and inves- tigate the ability to form organic 1D-structures by exposing it to two different molecules, i.e., tris{2,5-bis(3,5-bis-trifluoro- methyl-phenyhl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12-hexaazatriphenyl- ene (THAP) and 9,10-phenanthrenequinone (PQ). The results are a non-selective adsorption of THAP across the SiNWs/Ag(110) sam- ple, but a clearly selective adsorption of PQ on the SiNWs, leading to the formation of 1D organic structures. In addition, the elec- tronic structure of the NWs undergoes a significant transition from metallic to semi-conducting upon adsorption of 4 L of PQ. STM and scanning tunneling spectroscopy (STS) studies of the adsorption of PQ and THAP on the SiNWs were performed in a ser- ies of interconnected ultra-high vacuum (UHV) chambers (base pressure p $ 5 Â 10 À11 Torr) equipped with sputtering and anneal- ing facilities, a Si evaporator for SiNWs formation, organic mole- cules sublimation stations and a room-temperature Omicron STM. STM images were recorded in constant current topographic mode and processed with the WSxM software [5]. We present the STS results as both the current–voltage I(V) and the normalized differential conductance (dI/dV)/(I/V) curves, the latter serving as a good approximation of the local density of states (LDOSs) [6–8]. A single crystal Ag(110) purchased from Mateck was prepared by several cycles of Ar-ion sputtering (500 eV) and annealing (690 K). This procedure produced an atomically clean surface with 80 nm wide terraces exhibiting a rectangular (1 Â 1) Ag unit cell. The NWs were obtained by deposition of Si atoms on the clean Ag(110) surface at room-temperature at a typical rate of 0.7 Å/ min. This process led to the formation of a 1D-template surface consisting of SiNWs oriented along the ½  110 direction of the Ag(110) surface. The THAP molecules, synthesized and purified by the group of Marder [9], were deposited on the SiNWs/ Ag(110) surface by thermal evaporation ($480 K) from a quartz crucible at a rate of 0.5 Å/min. Deposition rate and thickness were estimated using a calibrated quartz crystal microbalance. The 0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.04.023 * Corresponding author. Tel.: +1 609 258 3582; fax: +1 609 258 6279. E-mail address: esalomon@princeton.edu (E. Salomon). Surface Science 602 (2008) L79–L83 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc THAP source was well out-gassed prior experiment and has been used several times for evaporation on various surfaces resulting in the formation of very nice, clean and well characterized THAP monolayers [10,11]. The PQ molecules, purchased from Sigma–Al- drich, were loaded under nitrogen atmosphere in a glass ampoule connected to a UHV leak valve. This setup was then mounted on the UHV system connected to the growth chamber. The exposure of the sample to PQ was estimated via the background pressure of molecules monitored with an ion gauge. The PQ exposure is gi- ven in Langmuir (1 L = 1.10 À6 Torr s). Fig. 1a and b displays two STM images of the 1D-template sur- face, which consists of multiple SiNWs adsorbed on Ag(110). The observed SiNWs are aligned along the ½  110 direction of the Ag sur- face and present identical characteristic width of 15 ± 1 Å. The structural and electronic properties of these SiNW structures have been thoroughly investigated by STM and PES [1–3]. However, to date no experimental data of the LDOSs had been reported. The dotted red curve in Fig. 2 1 shows the normalized differential con- ductance (dI/dV)/(I/V) obtained by STS on the SiNWs. The zero on the bias axis corresponds to the Fermi level (E F ). With negative and positive voltages applied to the sample, STS probes the local density of occupied and unoccupied states of the sample, respec- tively. The data exhibit two clear peaks located at À0.89 V and +1.82 V. These features correspond to two electronic quantum states presumably due to the confinement of electrons inside the SiNWs. The peak at À0.89 V is in good agreement with the first discrete state previously observed by PES at a binding energy of 0.92 eV with re- spect to E F [1]. A comparison of the STS data with DFT calculations shows that the observed LDOS is mainly due to the Si3p orbitals of the silicon atoms [4]. In addition, a non-zero LDOS is observed in the proximity (À2 eV to +2 eV) of E F . This non-zero LDOS, as well as the monotonic increase in current with bias (inset of Fig. 2), sug- gest that the SiNWs present a metallic character. This supports the observation made by Leandri et al. based on a detailed analysis of the Si2p core levels and valence band of the surface [1]. To address the issue of whether or not any type of molecule will react exclusively with the SiNWs, we used two different types of molecules. For the control experiment, a molecule (THAP) with no particular functional group that could specifically react with the SiNWs was chosen. Subsequently, a molecule (PQ) with a dicar- bonyl group, for which a strong and preferential reaction with the SiNWs is expected, was selected. The THAP molecule has been thoroughly studied in our group by means of PES and STM. Its adsorption on a clean Ag(110) substrate was characterized by STM [10], allowing us to compare it to the adsorption on the SiNWs modified Ag(110) surface. Two filled states STM images of the surface following the evap- oration of 4 Å of THAP are displayed in Fig. 3a and b. Each molecule appears as a six-pronged shape with six bright lobes and a dark center. The six-pronged shape is different from the shape usually observed for ordered layers of THAP adsorbed on Ag(110) or Au(111), where each molecule presents three bright lobes [10,11]. The discrepancy could be due to a different molecule–sub- strate interaction or to the fact that STM gives an average image of a molecule freely rotating at room-temperature between two equivalent positions. Tip effects cannot be discounted either. How- ever, regardless of the appearance of the molecule, the STM micro- graphs clearly show that the molecules are randomly adsorbed on the surface, some on top on the SiNWs and some in between two SiNWs. This non-selective behavior suggests that no specific inter- action takes place between the molecules and the SiNWs. In addi- tion, the THAP adsorbed on the clean Ag areas are not ordered according to the same structure as that observed upon adsorption on a clean Ag(110) surface [10]. The reactivity of the Ag surface is presumably locally modified by the SiNWs, possibly by the forma- tion a 2D surface Si–Ag alloy, as in the case of Si adsorbed on Cu(110) [12]. Furthermore, the long-range point-on-line coinci- dence observed between the THAP and Ag lattices on the THAP- on-clean-metal surface [10] , which is believed to be at the origin of the molecular ordering, cannot develop here between closely spaced SiNWs. Measurements performed one day after the evapo- ration of THAP on the surface exhibit the same random adsorption of the THAP molecules on the surface. This suggests that even after a significant amount of time for diffusion of THAP molecules is gi- ven, the adsorption remains non preferential. Having established that no preferential adsorption of THAP on the SiNWs is observed, we proceeded by exposing the SiNWs/ Ag(110) surface to 1 L of PQ, the result of which is shown in Fig. 4a. Two different types of structures are observed: bright pro- trusions, which are attributed to aggregates of PQ molecules, and faint lines, which correspond to the SiNWs. The faintness of the SiNWs, as compared to Fig. 1, is due to the presence of the molec- ular aggregates on top of the SiNWs which decreases the contrast between the SiNWs and the Ag surface. The molecular aggregates form discontinuous lines oriented along the ½  110 direction of the Ag that corresponds to the direction of the SiNWs. Furthermore, from this image one can notice that the molecular aggregates are exclusively adsorbed on top of the SiNWs. This observation is even clearer on the bottom right inset image. For this exposure, no molecular aggregate can be observed in between two SiNWs, clearly demonstrating a highly selective reaction between the PQ molecules and the SiNWs. As the exposure to PQ increases up to 4L(Fig. 4b), the 1D alignment and the selective adsorption of the organic structure are preserved, confirming that the first step in the adsorption process of the PQ on the SiNWs/Ag(110) surface is the specific interaction with the SiNWs. We propose that the selective adsorption of PQ molecules on the SiNWs occurs via chemical reaction. In order to describe the adsorption mechanism, we refer to the structural model of an indi- vidual SiNW proposed by Sahaf et al. [2]. According to this model, the top layer of the NWs consists of Si dimers (Fig. 5a). Following the experimental work done by Fang et al. and Hacker et al. [13,14] as well as the first principle calculations performed by Her- mann et al. [15] on the adsorption of PQ on Si(001), we propose that an interaction occurs here between the Si dimers and the dicarbonyl group of the molecule via a specific chemical reaction so-called [4 + 2] cycloaddition. Such a reaction has been previously observed and well described between molecules containing a dicarbonyl or diene functions and Si surface dimers [16–20]. This process is depicted in Fig. 5b and results in the formation of cova- Fig. 1. (a) Chemical structure of the THAP molecule and R stands for a trifluoro- methyl group (–CF 3 ). (b) Chemical structure of the PQ molecule. 1 For interpretation of color in Figs. 2–6, the reader is referred to the web version of this article. L80 E. Salomon, A. Kahn / Surface Science 602 (2008) L79–L83 SURFACE SCIENCE LETTERS lent bonds between the top silicon atoms of the NWs and the oxy- gen atoms of the molecules. These bonds are particularly interest- ing because they provide strong connectivity between the organic compound and the inorganic substrate (see Fig. 6). The electronic properties of the 1D organic nanowires (ONWs) obtained upon exposure of the SiNWs to 4 L of PQ are investigated via STS. Fig. 2 displays the normalized differential conductance re- corded on the pristine SiNWs (dotted red curve) and on the ONWs (full blue curve). We also have superimposed the LDOS measured in-between the SiNWs (black open circle curve) in order to show that the LDOS of the SiNWs is distinct from that of the Ag(110) nearby. Data carried out of both the SiNWs and the ONWs present significant changes. First, the current measured on the ONWs is substantially lower than that measured on the SiNWs, as expected from the relatively low conductivity of the organic molecules. Sec- ond, as inferred from both the I(V) (inset of Fig. 2) and (dI/dV)/(I/V) curves, the ONWs do not exhibit a metallic character. The plateau observed in the I(V) curve is clear evidence of the semi-conducting nature of the ONWs. The 0.3 eV gap observed in the (dI/dV)/(I/V) curve is consistent with the semi-conducting character of the ONWs, even though the gap appears smaller than expected for the PQ molecule ($2 eV for an isolated molecule [15]). Since the thickness of the ONWs is of the order of several ångström, the observed LDOS is a convolution of the LDOS of the SiNWs and that of the adsorbed PQ aggregates. Therefore, the contribution of the LDOS of the SiNWs at the proximity of the Fermi level is presumably Fig. 2. STM filled states images of the SiNWs adsorbed on Ag(110) (I t = 0.35 nA, V s = À50 mV) for (a) 50 Â 50 nm and (b) 20 Â 20 nm. Fig. 3. Average STS spectra representing the normalized differential conductivity of both the SiNWs (dotted red line) and the ONWs (full blue line) obtained upon the adsorption of 4 L of PQ. The inset shows the corresponding I(V) curves. Fig. 4. STM filled states images of the adsorption of 4 Å of THAP on the SiNWs/Ag(11 0) surface (I t = 0.35 nA, V s = À1.29 V) for (a) 110 Â 110 nm and (b) 22 Â 22 nm. E. Salomon, A. Kahn / Surface Science 602 (2008) L79–L83 L81 SURFACE SCIENCE LETTERS responsible for the shrinkage of the gap. For the same reason, the onset of the feature appearing at À0.5 V is presumably due to the quantum state previously observed for the SiNWs. Lastly, the new electronic structures observed at higher sample bias (>|1.5| V) are attributed to the electronic levels of the PQ molecules adsorbed on top of the SiNWs. The filled states region (negative bias) of the LDOS is in a good agreement with the PES, data published by Hacker and Hamers [14], in the case of PQ adsorbed on Si(00 1). In summary, we presented here a novel approach to form 1D or- ganic structures based on the adsorption of organic molecules on silicon nanowires. We demonstrated that carefully chosen organic compounds, such as dicarbonyl derivatives, selectively react with the silicon nanowires and form 1D organic nanowires along the same direction. We propose that the adsorption process occurs be- tween the Si dimers of the nanowires and the dicarbonyl groups of the molecules via a [4 + 2] cycloaddition, resulting in the formation of strong covalent bonds at the organic–inorganic interface. Finally, the STS measurements show that the electronic properties of the SiNWs can be selectively modified and functionalized depending on the adsorbed organic material. Acknowledgments Support of this work by the National Science Foundation (DMR- 0705920) and the Princeton MRSEC of the National Science Foun- dation (DMR-0213706) are gratefully acknowledged. We thank the group of Prof. S. Marder, Georgia Institute of Technology, for providing the THAP molecules, and Prof. J. Schwartz, Chemistry Department at Princeton University, for fruitful discussions and suggestions. Fig. 5. (a) About 90 Â 90 nm STM filled state images of the SiNWs/Ag(110) surface exposed to 1 L of PQ (I t = 0.33 nA, V s = À1.10 V). The inset corresponds to a zoom-in of 25 Â 25 nm. (b) About 90 Â 90 nm STM filled state images of the SiNWs/Ag(11 0) surface exposed to 4 L of PQ (I t = 0.2 nA, V s = À1.50 V). Fig. 6. (a) Top view of the template SiNWs/Ag(1 10) surface corresponding to the structure proposed by Sahaf et al. [2] and (b) side view of the PQ/SiNWs adsorption mechanism. L82 E. Salomon, A. Kahn / Surface Science 602 (2008) L79–L83 SURFACE SCIENCE LETTERS Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.susc.2008.04.023. References [1] C. Leandri, G. Le Lay, B. Aufray, C. Girardeaux, J. Avila, M.E. Da ´ vila, M.C. Asensio, C. Ottaviani, A. Cricenti, Surf. Sci. 574 (2005) L9. [2] H. Sahaf, L. Masson, C. Leandri, B. Auffray, G. Le Lay, F. Ronci, Appl. Phys. Lett. 90 (2007) 263110. [3] M.A. Valbuena, J. Avila, M.E. Davila, C. Leandri, B. Aufray, G. Le Lay, M.C. Asensio, Appl. Surf. Sci. 254 (2007) 50. [4] G M. He, Phys. Rev. B 73 (2006) 035311. [5] I. Horcas, R. Fernandez, J.M. Gomez-Rodriguez, J. Colchero, J. Gomez-Herrero, A.M. Baro, Rev. Sci. Instrum. 78 (2007) 013705. [6] J.A. Kubby, J.E. Griffith, R.S. Becker, J.S. Vickers, Phys. Rev. B 36 (1987) 6079. [7] R.M. Feenstra, Phys. Rev. Lett. 63 (1989) 1412. [8] G. Garreau, S. Hajjar, G. Gewinner, C. Pirri, Phys. Rev. B 71 (2005) 193308. [9] S. Barlow, Q. Zhang, B.R. Kaafarani, C. Risko, F. Amy, C.K. Chan, B. Domercq, Z.A. Starikova, M.Y. Antipin, T.V. Timofeeva, B. Kippelen, J L. Brédas, A. Kahn, S.R. Marder, Chem. – Eur. J. 13 (2007) 3537. [10] E. Salomon, Q. Zhang, S. Barlow, S.R. Marder, A. Kahn, J. Phys. Chem. C, submitted for publication. [11] S.D. Ha, Q. Zhang, S. Barlow, S.R. Marder, A. Kahn, Phys. Rev. B 77 (2008) 085433. [12] C. Polop, C. Rojas, J.A. Martin-Gago, R. Fasel, J. Hayoz, D. Naumovic, P. Aebi, Phys. Rev. B 63 (2001) 115414. [13] L. Fang, J. Liu, S. Coulter, X. Cao, M.P. Schwartz, C. Hacker, R.J. Hamers, Surf. Sci. 514 (2002) 362. [14] C.A. Hacker, R.J. Hamers, J. Phys. Chem. B 107 (2003) 7689. [15] A. Hermann, W.G. Schmidt, F. Bechstedt, J. Phys. Chem. B 109 (2005) 7928. [16] R. Konecny, D.J. Doren, J. Am. Chem. Soc. 119 (1997) 11098. [17] J.A. Barriocanal, D.J. Doren, J. Am. Chem. Soc. 123 (2001) 7340. [18] A.V. Teplyakov, M.J. Kong, S.F. Bent, J. Am. Chem. Soc. 119 (1997) 11100. [19] M.A. Filler, S.F. Bent, Prog. Surf. Sci. 73 (2003) 1. [20] S.F. Bent, Surf. Sci. 500 (2002) 879. E. Salomon, A. Kahn / Surface Science 602 (2008) L79–L83 L83 SURFACE SCIENCE LETTERS . Surface Science Letters One-dimensional organic nanostructures: A novel approach based on the selective adsorption of organic molecules on silicon nanowires Eric. molecules on the nanowires leads to a well-defined one-dimensional aggregation and changes the metallic character of the nanowires to a semi-conducting one. Ó