Ab InitioStudy of functionalized 1 nm Silicon Nanoparticles

Journal of Physics: Conference Series Related content OPEN ACCESS Ab Initio Study of functionalized 1 nm Silicon Nanoparticles To cite this article: M E Ahmed et al 2010 J. Phys.: Conf. Ser. 245 012046 - Topical Review James R Chelikowsky, Leeor Kronik and Igor Vasiliev - Computational design of in vivo biomarkers Bálint Somogyi and Adam Gali - Ab initio Optical Absorption in Conjugated Polymers: the Role of Dimensionality Alice Ruini View the article online for updates and enhancements. Recent citations - Surface-engineered silicon nanocrystals Davide Mariotti et al This content was downloaded from IP address 3.236.55.199 on 10/06/2020 at 03:08 Quantum Dots 2010 Journal of Physics: Conference Series 245 (2010) 012046 IOP Publishing doi:10.1088/1742-6596/245/1/012046 Ab Initio Study of functionalized 1 nm Silicon Nanoparticles M. E. Ahmed, J. P. Goss, R. J. Eyre, P. R. Briddon, M. A. Taylforth School of Electrical, Electronic and Computer Engineering, Newcastle University, Newcastle upon Tyne, NE1 7RU, England, United Kingdom E-mail: [email protected] Abstract. Functionalization of silicon nanoclusters reveals electronic and optical phenomena that can be utilized in a range of applications, including optical sensing, biological imaging and optoelectronic devices. Using density functional theory calculations, light-absorption and luminescence processes are modelled at the quantum mechanical level. In this study, a number of chemical functional groups are attached to the surface of silicon quantum dots (Si-QDs) of ∼1 nm diameter via either C4 H8 or C8 H16 alkane chains. In these results, the impact of the functional groups upon the electronic structure and optical absorption spectra of composite systems depend upon the chemical nature of the functional group and the alkane chain length. 1. Introduction In recent years, silicon nanocrystalline materials have become an interesting area in physics, chemistry, material science and biophysics. Various applications have resulted in biomedical applications, semiconductor nanomaterials and quantum dot devices[1, 2]. According to experimental observations and theoretical predictions, the properties of large Si-QDs are close to bulk silicon material in terms of the energy gap and emission wave length [3]. However, when Si-QDs have a diameter less than 5 nm, PL emission is in the visible range with high intensity. Quantum confinement is a key factor, but because there are up to around 50% of the atoms at the surface, the role of surface states is likely to be important. Termination using hydrocarbon chains is an effective way of stabilizing the surface against oxidation [4]. Furthermore, functionalization of Si-QDs with different chemically active molecules is currently being investigated for possible applications as luminescent labels in biological applications, since the intense luminescence of Si-QDs are at wavelengths where biological systems do not absorb strongly. Theoretically, the role of surface functional groups with regards to their optical properties is an area still in development. For complete coverage, as the polarity of the Si-QD interface increases, the optical band-gap becomes increasingly affected by charge transfer at the interface rather than by quantum confinement [5]. Previous studies of surface functionalization largely involved functional groups bonded directly to the Si-QD rather than at the end of the long alkane chains. In this paper we present a quantum chemical study into the impact upon the electronic and optical properties of Si-QDs for various functional groups which terminate the alkane chains bonded to the Si-QD surface. c 2010 IOP Publishing Ltd  1 Quantum Dots 2010 Journal of Physics: Conference Series 245 (2010) 012046 IOP Publishing doi:10.1088/1742-6596/245/1/012046 2. Computational Method Calculations were carried out using the density functional technique, which is implemented in AIMPRO code (ab initio modelling program) [6, 7]. All calculations presented here use the generalized gradient approximation [8] for total energies and forces, and core electrons are treated using norm-conserving pseudo potentials [9]. The Si-QD is modeled starting from tetrahedral, hydrogen-terminated cluster, Si35 H36 , which is illustrated in Fig. 1. The non-functionalized cluster was first structurally relaxed, and the resulting geometry analyzed for its electronic structure and optical absorption. The main features of the electronic structures are the energies of the electronic-states, which represent the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs). Optical absorption spectra are obtained from the calculated complex dielectric function [10]. Each electron state is assigned an width using a polynomial broadening, characterized by a width of 0.1 eV. For each functional group, one of the surface hydrogen atoms of the Si-QD is substituted by a linear alkane chain, and the functional group is attached to the other end of this chain. In all cases there are either four or eight methylene units (CH2 ) between the “functional group” and the Si-QD, as shown in Fig. 1. The functional groups explored in this study are listed in table 1. To determine how significant the impact that functionalization has upon electronic and optical properties of the Si-QDs, in addition to varying the chemical functional group, we change the length of alkane chain from C4 H8 to C8 H16 with every functional group. Figure 1. Schematic showing the fully-hydrogenated silicon dot Si35 H36 , The left side is undecorated fully-hydrogenated silicon dot, while the right side is the Si-QD after the functionalization with a single functional group. Pink, white, grey and red atoms are silicon, hydrogen, carbon and oxygen, respectively. 3. Results Calculations for Kohn-Sham energy levels, and the electronic spectra are given for functionalized and non-functionalized Si-QD systems. They can be viewed from the HOMOs and LUMOs energy levels with the associated optical gap, as illustrated in table 1. The results divided into three main groups depending upon the electronic states introduced into the optical gap. Firstly, group(A) which contains alcohol, alkane, alkene, nitride, carbonic acid chloride, and halogens functional groups. As in table 1, in this group(A), the attached functional groups to the Si-QDs introduce no new electronic state into the optical gap. Fig. 2 2 Quantum Dots 2010 Journal of Physics: Conference Series 245 (2010) 012046 IOP Publishing doi:10.1088/1742-6596/245/1/012046 0.0 -1.0 -1.0 -1.0 -2.0 -2.0 -2.0 -3.0 -3.0 -3.0 -4.0 -4.0 -4.0 -5.0 -6.0 (C) Energy (eV) 0.0 Energy (eV) (B) 0.0 Energy (eV) (A) -5.0 -6.0 -5.0 -6.0 -7.0 -7.0 -7.0 -8.0 -8.0 -8.0 -9.0 -9.0 -9.0 -10.0 -10.0 -10.0 (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) Figure 2. The Kohn-Sham levels for 1 nm Si-QD of (A), (B) and (C) for alkane, carbonicacid and aldehyde respectively, when (a), molecule with four carbon chain length (b) Si-QD attached to four carbon chain length, (c) undecorated H-terminated Si-QD, (d) molecule with eight carbon chain length and (d) molecule with eight carbon chain length. The shaded regions indicate the position of the bands for undecorated Si-QD (c). Optical absorption coefficient (arb. units) Optical absorption coefficient (arb. units) 100000 10000 1000 100 10 1 2.0 2.5 3.0 3.5 4.0 4.5 Energy (eV) 5.0 5.5 6.0 (C) Optical absorption coefficient (arb. units) (B) (A) 100000 10000 1000 100 10 1 2.0 2.5 3.0 3.5 4.0 4.5 Energy (eV) 5.0 5.5 6.0 100000 10000 1000 100 10 1 2.0 2.5 3.0 3.5 4.0 4.5 Energy (eV) 5.0 5.5 6.0 Figure 3. Plot of the calculated optical absorption spectra corresponding to the undecorated hydrogen terminated Si-QD (black full line) in three graphs, (A) alkane functional group on silicon quantum dot (purple line), (B) carboxylic acid functional group (red line) and (C) aldehyde functional group (dashed green line). (A) is represents alkane functional group Kohn-Sham energy levels as an example of this group. From the plot we can see that the chain of eight alkane chain has no new electronic state in the gap. This confirms that the length of alkane chain does not affect the states in the gap. At the same time, from Fig.3(A), the optical absorption spectrum for the same alkane functional group, produces no difference in terms of optical absorption coefficient compared to the undecorated Si-QD. Secondly in group (B), which includes alkyne, arene and carboxylic acid. Shallow gap-states are introduced just above the HOMO of the Si-QD. Carboxylic acid functional group (carboxylic acid contains a carbon-oxygen double bond and carbon-oxygen-hydrogen single bonds) is chosen as an illustration to represent this group in Fig. 2 (B). From Fig. 3 (B), we can see a minor shift in absorption spectra. Finally, group(C) which contains amide, thiole, amine, aldehyde, sulfide and ferrocene. We can see deep electronic states introduced into the optical gap. As a clear example, Fig. 2 (C) shows the electronic structure of aldehyde functionalized Si-QD (aldehyde contains a carbonoxygen double bond). Here, a electronic state localised on the functional group lies well above the HOMO of the Si-QD. In Fig. 2 (C) we can see that the electronic state in HOMO-LUMO 3 Quantum Dots 2010 Journal of Physics: Conference Series 245 (2010) 012046 IOP Publishing doi:10.1088/1742-6596/245/1/012046 gap corresponding with optical energy shift in the optical absorption by eV in Fig. 3 (C). Table 1. Electronic structure data contain HOMO and LUMO energies for various functional groups attaches to the Si-QD via (C4 H8 and C8 H16 ). HOMO and LUMO energies are in eV relative to the vacuum level at zero. The groups are ranked by increasing energy of the HOMO and divided into the three groups as indicated in the text. C4 H8 C8 H16 Category Family Functional group HOMO LUMO HOMO LUMO Pure QD -2.39 -6.03 Nitrile (CN) -6.10 -2.49 -6.05 -2.44 Carbonic acid chloride (COCl) -6.06 -2.44 -6.02 -2.41 Chloride (Cl) -6.04 -2.43 -6.01 -2.39 Bromide (Br) -6.03 -2.42 -6.00 -2.39 A Fluoride (F) -6.03 -2.42 -6.00 -2.39 Alcohol (OH) -5.99 -2.38 -5.95 -2.37 Alkene (C2 H3 ) -5.98 -2.37 -5.94 -2.37 Alkane (H) -5.97 -2.36 -5.96 -2.35 Alkyne (C2 H1 ) -5.78 -2.38 -5.73 -2.37 Arene (C6 H5 )) -5.66 -2.37 -5.61 -2.37 B Carboxylic acid (C(OH)O) -5.62 -2.38 -5.55 -2.36 Amide (CONH2 ) -5.51 -2.40 -5.45 -2.39 Thiol (SH) -5.50 -2.41 -5.45 -2.40 Amine (N2 ) -5.44 -2.37 -5.36 -2.35 C Aldehyde (CHO) -5.11 -2.39 -5.04 -2.38 Sulfide (SCH3 ) -5.02 -2.40 -4.96 -2.39 Ferrocene ((C5 H5 )2 Fe) -4.07 -2.37 -4.01 -2.37 4. Discussion and Conclusions The presence and location of electronic states in the band-gap of the 1 nm Si-QD is strongly dependent upon the chemical nature of the functional group. 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