Papers

AVS2002 Session SS+EL+OF-ThM: Reactions and Patterning of Organics on Silicon

Thursday, November 7, 2002 8:20 AM in Room C-112C

Thursday Morning

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8:20 AM		SS+EL+OF-ThM-1 Fixation of Alkyl Groups on Si(111) Surface through C-Si Single Covalent Bond formed by Reaction of Grignard Reagent and H:Si(111) T. Yamada, T. Inoue, K. Yamada, N. Takano, T. Osaka (Waseda University, Japan); H. Harada, K. Nishiyama, I. Taniguchi (Kumamoto University, Japan) Alkyl adsorbates that are directly bonded to the outermost atoms of silicon wafer surfaces have prospective properties for application in nanometer-scale fabrication and surface functionalization.¹ Several methods have been proposed to deposit alkyl groups on hydrogen-terminated H:Si(111).² The conversion of the H-Si bonds in H:Si(111) into C-Si bonds caused by chemical processes is an important issue in preparing the organic adlayers. In this work, high-resolution electron energy loss spectroscopy (HREELS) was utilized to prove the formation of single covalent bonds between Si(111) surface atoms and alkyl groups by the chemical reaction of a Grignard reagent and hydrogen-terminated H:Si(111)(1x1).² The reaction was performed by heating a piece of H:Si(111) in 1M tetrahydrofuran (at 65°C) or diethylether solution (at 30°C) of desired alkylmagnesium halide for 18 hours under Ar atmosphere. By this reaction condition, somewhat 20% of the product surface were still covered with residual hydrogen. The bending vibration mode of the residual hydrogen (630 cm^-1) obscured the alkyl signals in the range of 600 - 700 cm^-1. By using deuterium-terminated D:Si(111), the vibration at 680 cm^-1, assigned to the C-Si bond, was isolated within the spectrum of CH₃-. The CH₃ groups were thermally stable at temperatures below 600 K. Similar features were observed for C₂H₅-, phenyl- and so on. The C-Si bonds are essential for enhancing the stability and lowering the mobility of alkyl moieties. Such properties of alkyl moieties of will lead to a new prospective science and technology in nanometer-scale fabrication. ¹T. Yamada, N. Takano, K. Yamada, S. Yoshitomi, T. Inoue, and T. Osaka, Jpn. J. Appl. Phys. 40 (2001) 4845. ²R. Boukherroub, S. Morin, F. Bensebaa and D. D. M. Wayner, Langmuir 15 (1999) 3831.
8:40 AM		SS+EL+OF-ThM-2 Chemomechanical Production of Sub-Micron Edge Width, Functionalized, ~20 Micron Features on Silicon M.R. Linford, Y.-Y. Lua, T.L. Niederhauser, B.A. Wacaser (Brigham Young University); I.A. Mowat (Charles Evans & Associates); A.T. Woolley, R.C. Davis (Brigham Young University); H.A. Fishman (Stanford University Medical School) We have recently reported that monolayers on silicon can be formed, and silicon substrates concomitantly patterned, when native oxide-terminated silicon is scribed with a diamond-tipped instrument in the presence of 1-alkenes,^1,2 1-alkynes,^1,2 alkyl halides (chlorides, bromides, and iodides),^2,3 and alcohols.^3,4 Monolayers were prepared in the open laboratory with reagents that had not been degassed.^1-4 However, while this method is particularly facile, the features produced using a diamond scribe are coarse and irregular.¹ The high degree of edge and surface roughness in these features will no doubt limit the utility of our earlier work in some circumstances. Here we describe a substantial improvement in our earlier method by showing the production of sharp, well-defined, functionalized features on silicon that are invisible to the naked eye and very shallow. In contrast to the earlier procedure that used oxide-terminated silicon and a diamond-tipped scribe,^1-4 this new method consists of 1) cleaning and drying a silicon shard, 2) immersing the silicon in a fluoride ion etch to remove its native oxide and produce hydrogen-terminated silicon, 3) wetting the dry, H-terminated silicon surface with a reactive liquid, and 4) scribing the surface with a small tungsten carbide ball. Both Si(100) and Si(111) were successfully patterned and functionalized with this new method. ¹ Niederhauser, T. L.; Jiang, G.; Lua, Y.-Y.; Dorff, M. J.; Woolley, A. T.; Asplund, M. C.; Berges, D. A.; Linford, M. R. Langmuir 2001, 19, 5889-5900. ² Lua, Y.-Y.; Niederhauser, T. L.; Matheson, R.; Bristol, C.; Mowat, I. A.; Asplund, M. C.; Linford, M. R. In Press Langmuir 2002. ³ Niederhauser, T. L.; Lua, Y.-Y.; Sun, Y.; Jiang, G.; Strossman, G. S.; Pianetta, P.; Linford, M. R. Chem.Mater. 2002, 14, 27-29. ⁴ Niederhauser, T. L.; Lua, Y.-Y.; Jiang, G.; Davis, S. D.; Matheson, R.; Hess, D. A.; Mowat, I. A.; Linford, M. R. In press Angewandte Chemie 2002.
9:00 AM		SS+EL+OF-ThM-3 Formation of Nanoscale Organic and Inorganic Features on Semiconductor Surfaces J.M. Buriak (Purdue University) Integration of molecular devices and nanoscale materials with semiconductors, including silicon and germanium, is an area of intense interest, due to the potential for interfacing nanomaterials with the macroworld. We have developed a number of wet chemical routes which allow for covalent attachment of both organic functionalities, including molecular wires, and inorganic nanoparticles. For instance, a cathodic electrografting reaction between alkynes and hydride-terminated silicon surfaces results in alkynyl moieties bound directly through Si-C bonds, with no intervening oxide layer. The surfaces are air and water stable, and can withstand boiling pH 12 solutions. In order to pattern these alkynyl groups on the silicon surface in nanoscale regions, conducting probe lithography has been utilized to write the organic monolayers, with feature sizes as small as 30 nm. For inorganic structure patterning, electroless deposition has been combined with microcontact printing, dip pen nanolithography (DPN), and UV-mediated hydrometallation to produce nano- and micron-scale features. These approaches and others will be described.
9:40 AM		SS+EL+OF-ThM-5 Structures, Dynamics, and Chemical Reactivity of Si (001) at Finite Temperatures: A First Principles Study D. Pillay, Y. Wang, G.S. Hwang (The University of Texas at Austin) Imparting organic functions onto a well-defined functionality of semiconductor surfaces with atomic-scale precision provides an enormous opportunity to develop new molecular devices including chemical and biological sensors and molecular electronic devices. For semiconductor systems, the rates and pathways of chemical reactions are strongly influenced by local electronic structures determined by surface reconstructions and defects. Due to such complex structural effects, chemical dynamics on semiconductor surfaces has not been fully understood. To gain molecular-level control, therefore, first we must develop a detailed understanding of structures, dynamics, and chemical reactivity of clean, defective, or modified (with various adsorbates) surfaces at finite temperatures, along with the structures and bonding of organic compounds onto the surfaces. In this talk we will present first principles quantum mechanics [Density Functional Theory with plane-wave basis sets and pseudopotentials] simulations for structures, dynamics, and chemical reactivity (towards organic species) of (001)-faced Si and Ge surfaces at finite temperatures. This includes the dynamics of buckled dimers on clean and defective surfaces and their effects on adsorption dynamics of various organic molecules.
10:00 AM		SS+EL+OF-ThM-6 Superexchange Interactions in STM-Organic-Semiconductor Systems L.C. Teague, J.J. Boland (University of North Carolina at Chapel Hill) The incorporation of organic layers and individual molecules into existing semiconductor technologies requires a thorough understanding of surface/molecule reactions. Although numerous studies have focused on the reaction of individual molecules with the Si(100) surface, the specific reaction mechanisms remain poorly understood. The similarity of the Si(100)-2 x 1 surface chemistry with that of C=C systems opens up the possibility of a wide range of organic chemistry reactions. Here, a combination of Scanning Tunneling Microscopy (STM) and Density Functional Theory (DFT) calculations are used to study and interpret the reaction of 1,3-cyclohexadiene (1,3-CHD) with the bare Si(100)-2 x 1 surface. Because STM probes the local density of states (LDOS), the local bonding geometry of 1,3-CHD can be inferred from the location of the π bond in the adsorbed molecule. Other groups have reported similar observations.^1,2 However, DFT calculations indicate the π* orbital is located several eV above the Fermi Energy and should be energetically inaccessible under typical bias conditions. Here, we show that these images can be understood by considering the interaction between the STM tip and the molecule-surface system. The superexchange mixing of the tip dangling bond state with the filled π state on the molecule produces a new state within the tunneling window. This state is responsible for the observed image contrast and suggests superexchange effects of this type may be important in understanding the charge transfer that occurs through these molecular systems. ¹ Hamaguchi, K.; Machida, S.; Nagao, M.; Yasui, F.; Mukai, K.; Yamashita, Y.; Yoshinobu, J.; Kato, H. S.; Okuyama, H.; Kawai, M.; Sato, T.; Iwatsuki, M. J. Phys. Chem. B 2001, 105, 3718. ² Hovis, J. S.; Liu, H.; Hamers, R. J. J. Phys. Chem. B, 1998, 102, 6873.
10:20 AM		SS+EL+OF-ThM-7 Modifying the Semiconductor Interface with Organonitriles M.A. Filler, C. Mui, C.B. Musgrave, S.F. Bent (Stanford University) Organic functionalization of group-IV semiconductor surfaces has recently garnered considerable attention and applications in the areas of molecular electronics, biological recognition, and reagentless micropatterning have been proposed. If these and other concepts are to become technologically feasible, however, the creation of an ordered and selectively grown layer as well as the ability to successively attach additional organic monolayers will be necessary. Organonitrile compounds were studied as potential candidates for first and subsequent layer surface reactions on Si(100)-2x1 and Ge(100)-2x1. Bonding is investigated experimentally with infrared spectroscopy and theoretically with density functional theory. We find that acetonitrile does not react on the Ge(100)-2x1 surface at room temperature and explain this result with kinetic and thermodynamic arguments. A [4+2] cycloaddition product through the conjugated π system and a [2+2] C=C cycloaddition product through the alkene are found to be the dominant surface adducts for the multifunctional molecule 2-propenenitrile. While the non-conjugated molecules 3-butenenitrile and 4-pentenenitrile are not expected to form a [4+2] cycloaddition product, both show vibrational modes characteristic of this adduct and we propose the possibility of a surface catalyzed reaction. Pathways directly involving only the nitrile functional group are thermodynamically unfavorable at room temperature on Ge(100)-2x1 and the conversion of the remaining nitrile functionality as well as its use in additional surface reactions will also be presented.
10:40 AM		SS+EL+OF-ThM-8 Adsorption and Reaction of Allyl- and Ethyl-amine on Germanium and Silicon Surfaces P. Prayongpan, C.M. Greenlief (University of Missouri-Columbia) The adsorption and reaction of allyl- and ethyl-amine with the Ge(100) and Si(100) surfaces is examined. These processes are followed by a variety of surface sensitive techniques including ultraviolet photoelectron spectroscopy and temperature programmed desorption. Possible adsorption structures are also examined by theoretical methods. Density functional theory calculations are used to help interpret the photoelectron spectroscopy data. The calculated molecular orbital energies (within Koopmans' approximation) are used to help identify adsorbed molecular species, as well as, reaction intermediates. The interaction of these nitrogen-containing molecules with surface dimmer bonds and ordering of the resulting surface layers will be discussed.
11:00 AM		SS+EL+OF-ThM-9 Adsorption Chemistry of Cyanogen Bromide and Iodide on Silicon (100) N.F. Materer, P. Rajasekar, E.B. Kadossov (Oklahoma State University) The adsorption of cyanogen iodide (ICN) and bromide (BrCN)on a silicon (100) surface is studied by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and thermal desorption spectroscopy (TPD). After submonolayer exposures, XPS indicates that the CN triple bond of both ICN and BrCN emains intact upon adsorption at 100K. The UPS spectrum of these molecules contains two peaks assigned to the pi electrons in the CN triple bond. The splitting of these levels, due to the interaction between the pi electrons on the cyanogen and the halide, show that some portion of the initially adsorbed cyanogens halide also remains intact upon low temperatures adsorption. In contrast, the UPS spectrum of ICN and BrCN adsorbed at room temperature on Si(100) contains only one peak due to the pi electrons in the carbon nitrogen triple bond. The lack of splitting in the room temperature UPS spectrum is a result of XC (X=Br, I) bond dissociation. Thus, the XC bonds breaks while the CN bond remains intact during room temperature adsorption on Si(100). Upon annealing the Si(100) surface to higher temperatures, the UPS spectra indicates that the C-N triple bond remains intact until approximately 700 K. Simultaneous changes in the C 1s photoelectron peak are consistent with the idea that C-N bond cleavage in the cyanogen halides is correlated with silicon carbide formation.
11:20 AM		SS+EL+OF-ThM-10 Theoretical Adsorption Studies of ICN on the Si(100) Surface E.B. Kadossov, P. Rajasekar, N.F. Materer (Oklahoma State University) Ab initio quantum calculations have been used to study the adsorption and surface reactions of ICN on the Si(100) surface represented by Si₂H₁₂ single-dimer cluster. At low temperatures, experiments show that some faction of the initially exposed ICN is molecularly absorbed to the surface. Calculations of the molecularly absorbed species support the formation of end-on configuration with the N forming a dative bond with the lone pair on the Si(100) surface. Upon annealing, experiments reveal that the remaining molecular adsorbed ICN species dissociate to produce I and CN species on the surface. Calculations show that this process can take place directly or through a stable side-on adsorption intermediate. The transition barrier to form the side-on intermediate species is slightly lower than for the direct reaction (12.82 vs. 17.26 kJ/mol). After dissociation, the CN is bound to the silicon surface through either the C or the N ends. The C bound species possesses the lowest energy and is consistent with experimental XPS results. In addition, these two possible structures are separated by an activation barrier of 107.39 kJ/mol, easily overcome by the excess adsorption energy. An alternate pathway for the ICN side-on species is to isomerize into an INC structure through a 212.26 kJ/mol activation barrier. However, the activation barrier between this new species and the dissociated state is 5.53 kJ/mol. This transition barrier is even lower than the 84.05 kJ/mol barrier between side-on ICN surface species and the dissociated state.
11:40 AM		SS+EL+OF-ThM-11 Infrared Study of Adsorption of C₆H₆ onto Si(100)(2x1) M. Shinohara, H. Watanabe, Y. Kimura, H. Ishii, M. Niwano (Tohoku University, Japan) The interaction of benzene with the (100) and (111) surfaces of silicon has proven to be an interesting model system for molecular adsorption on semiconductor surfaces. The adsorption of benzene on the Si(100)(2x1) surface has been expensively studied in recent years both experimentally and theoretically. Previous elaborate theoretical calculations predicted that adsorption of benzene onto the Si(100)(2x1) surface leads to two different adsorption structures: One corresponds to benzene adsorbed on top of a dimer row between two adjacent Si dimers. This structure has four C atoms of benzene bonded to four Si atoms from two adjacent dimers. The other corresponds to the benzene molecule that sticks on top of the dimer row and has two of its C atoms bonded to two Si atoms of a single surface dimer. However, there still is a controversy regarding which structure is more favored. In this study, we have investigated the adsorption of benzene C₂H₆ on the Si(100)(2x1) surface using infrared absorption spectroscopy (IRAS) in the multiple internal reflection geometry (MIR) and the so-called hybrid density-functional theory (DFT) to determine the most preferred adsorption structure of benzene on Si(100)(2x1) at room temperature. IRAS-MIR provides us with valuable information about the hydrogen bonding configurations on semiconductor surfaces. We analyzed IRAS spectra in the C-H stretching vibration region to determine the detailed adsorption structure of benzene. The central result is that benzene adsorbs in different manners depending on the surface coverage of benzene: at low coverage the molecule adsorb on the surface to favor the formation of benzene adsorbed on two adjacent dimers. On the other hand, at high coverage the molecule adsorb on the Si surface to generate benzene adsorbed on a single dimer. We also discuss the reason why the adsorption structure depends on surface coverage.