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
Trang 1Surface 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
a r t i c l e i n f o
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
a b s t r a c t
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 (1 1 0) surface
(Ag(1 1 0)) [1,2] This system has been thoroughly studied by
means of scanning tunneling microscopy (STM), low energy
elec-tron diffraction and photoelecelec-tron 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-phenanthrmethyl-phenyhl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12-hexaazatriphenyl-enequinone (PQ) The results are a
non-selective adsorption of THAP across the SiNWs/Ag(1 1 0) 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 1011Torr) 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(1 1 0) 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 1) Ag unit cell The NWs were obtained by deposition of Si atoms on the clean Ag(1 1 0) 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 ½110 direction of the Ag(1 1 0) surface The THAP molecules, synthesized and purified
by the group of Marder [9], were deposited on the SiNWs/ Ag(1 1 0) 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.
* Corresponding author Tel.: +1 609 258 3582; fax: +1 609 258 6279.
E-mail address: esalomon@princeton.edu (E Salomon).
Surface Science
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / s u s c
Trang 2THAP 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 = 1.106Torr s)
Fig 1a and b displays two STM images of the 1D-template
sur-face, which consists of multiple SiNWs adsorbed on Ag(1 1 0) The
observed SiNWs are aligned along the ½110 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 inFig 21shows 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 (EF) 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 0.89 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 0.89 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 EF[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 (2 eV to +2 eV) of EF This non-zero LDOS, as well
as the monotonic increase in current with bias (inset ofFig 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(1 1 0) substrate was characterized by
STM[10], allowing us to compare it to the adsorption on the SiNWs modified Ag(1 1 0) surface
Two filled states STM images of the surface following the evap-oration of 4 Å of THAP are displayed inFig 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(1 1 0) or Au(1 1 1), 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(1 1 0) 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(1 1 0) [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(1 1 0) 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 toFig 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 ½110 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
4 L (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(1 1 0) 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(0 0 1), 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 inFig 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
Trang 3lent 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 (seeFig 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 2displays 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(1 1 0) 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 ofFig 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(1 1 0) (I t = 0.35 nA, V s = 50 mV) for (a) 50 50 nm and (b) 20 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.
E Salomon, A Kahn / Surface Science 602 (2008) L79–L83 L81
Trang 4responsible for the shrinkage of the gap For the same reason, the
onset of the feature appearing at 0.5 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(0 0 1)
In summary, we presented here a novel approach to form 1D
or-ganic structures based on the adsorption of oror-ganic 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 90 nm STM filled state images of the SiNWs/Ag(1 1 0) surface exposed to 1 L of PQ (I t = 0.33 nA, V s = 1.10 V) The inset corresponds to a zoom-in of
25 25 nm (b) About 90 90 nm STM filled state images of the SiNWs/Ag(1 1 0) surface exposed to 4 L of PQ (I t = 0.2 nA, V s = 1.50 V).
Fig 6 (a) Top view of the template SiNWs/Ag(1 1 0) surface corresponding to the structure proposed by Sahaf et al [2] and (b) side view of the PQ/SiNWs adsorption mechanism.
Trang 5Appendix A Supplementary data
Supplementary data associated with this article can be found, in
the online version, atdoi: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