| 1. |
- Kislitsyn, Dmitry A., et al.
(författare)
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Communication : Visualization and spectroscopy of defects induced by dehydrogenation in individual silicon nanocrystals
- 2016
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 144:24
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Tidskriftsartikel (refereegranskat)abstract
- We present results of a scanning tunneling spectroscopy (STS) study of the impact of dehydrogenation on the electronic structures of hydrogen-passivated silicon nanocrystals (SiNCs) supported on the Au(111) surface. Gradual dehydrogenation is achieved by injecting high-energy electrons into individual SiNCs, which results, initially, in reduction of the electronic bandgap, and eventually produces midgap electronic states. We use theoretical calculations to show that the STS spectra of midgap states are consistent with the presence of silicon dangling bonds, which are found in different charge states. Our calculations also suggest that the observed initial reduction of the electronic bandgap is attributable to the SiNC surface reconstruction induced by conversion of surface dihydrides to monohydrides due to hydrogen desorption. Our results thus provide the first visualization of the SiNC electronic structure evolution induced by dehydrogenation and provide direct evidence for the existence of diverse dangling bond states on the SiNC surfaces. Published by AIP Publishing.
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| 2. |
- Kislitsyn, Dmitry A, et al.
(författare)
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Mapping of Defects in Individual Silicon Nanocrystals Using Real-Space Spectroscopy.
- 2016
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7:6, s. 1047-1054
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Tidskriftsartikel (refereegranskat)abstract
- The photophysical properties of silicon semiconductor nanocrystals (SiNCs) are extremely sensitive to the presence of surface chemical defects, many of which are easily produced by oxidation under ambient conditions. The diversity of chemical structures of such defects and the lack of tools capable of probing individual defects continue to impede understanding of the roles of these defects in SiNC photophysics. We use scanning tunneling spectroscopy to study the impact of surface defects on the electronic structures of hydrogen-passivated SiNCs supported on the Au(111) surface. Spatial maps of the local electronic density of states (LDOS) produced by our measurements allowed us to identify locally enhanced defect-induced states as well as quantum-confined states delocalized throughout the SiNC volume. We use theoretical calculations to show that the LDOS spectra associated with the observed defects are attributable to Si-O-Si bridged oxygen or Si-OH surface defects.
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| 3. |
- Kocevski, Vancho, 1984-, et al.
(författare)
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Anharmonic Vibrational Frequency Shifts upon Interaction of Phenol(+) with the Open Shell Ligand O2 : The Performance of DFT Methods versus MP2
- 2012
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 116:8, s. 1939-1949
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Tidskriftsartikel (refereegranskat)abstract
- Anharmonic vibrational frequency shifts of the phenol(+) O–H stretching mode upon complex formation with the open-shell ligand O2 were computed at several DFT and MP2 levels of theory, with various basis sets, up to 6-311++G(2df,2pd). It was found that all DFT levels of theory significantly outperform the MP2 method with this respect. The best agreement with the experimental frequency shift for the hydrogen-bonded minimum on the potential energy surfaces was obtained with the HCTH/407 functional (−93.7 cm–1 theoretical vs −86 cm–1 experimental), which is a significant improvement over other, more standard DFT functionals (such as, e.g., B3LYP, PBE1PBE), which predict too large downshifts (−139.9 and −147.7 cm–1, respectively). Good agreement with the experiment was also obtained with the mPW1B95 functional proposed by Truhlar et al. (−109.2 cm–1). We have attributed this trend due to the corrected long-range behavior of the HCTH/407 and mPW1B95 functionals, despite the fact that they have been designed primarily for other purposes. MP2 method, even with the largest basis set used, manages to reproduce only less than 50% of the experimentally detected frequency downshift for the hydrogen-bonded dimer. This was attributed to the much more significant spin contamination of the reference HF wave function (compared to DFT Kohn–Sham wave functions), which was found to be strongly dependent on the O–H stretching vibrational coordinate. All DFT levels of theory outperform MP2 in the case of computed anharmonic OH stretching frequency shifts upon ionization of the neutral phenol molecule as well. Besides the hydrogen-bonded minimum, DFT levels of theory also predict existence of two other minima, corresponding to stacked arrangement of the phenol(+) and O2 subunits. mPW1B95 and PBE1PBE functionals predict a very slight blue shift of the phenol(+) O–H stretching mode in the case of stacked dimer with the nearly perpendicular orientation of oxygen molecule with respect to the phenolic ring, which is entirely of electrostatic origin, in agreement with the experimental observations of an additional band in the IR photodissociation spectra of phenol(+)–O2 dimer [Patzer, A.; Knorke, H.; Langer, J.; Dopfer, O. Chem. Phys. Lett. 2008, 457, 298]. The structural features of the minima on the studied PESs were discussed in details as well, on the basis of NBO and AIM analyses.
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| 4. |
- Kocevski, Vancho, 1984-, et al.
(författare)
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Band alignment switching and the interaction between neighbouring silicon nanocrystals embedded in a SiC matrix
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91, s. 165429-165435
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Tidskriftsartikel (refereegranskat)abstract
- We present results from density functional theory of the electronic properties of silicon nanocrystals (Si NCs) embedded in a silicon carbide (SiC) matrix, considering different combinations of various NCs and host matrix sizes. We show that the NC and the host matrix form a type-II band alignment, with the states at the top of the valence band being in the Si NC and the states at the bottom of the conduction band in the host matrix. Moreover, this band alignment can be interchanged with introducing oxygen at the interface. We demonstrate that the charge densities of some valence band states can overlap with the charge densities of the neighbouring NCs. We also demonstrate that this leakage of states is significant when the distance between the neighbouring NCs is less than ~1.6 nm.
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| 5. |
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| 6. |
- Kocevski, Vancho, 1984-, et al.
(författare)
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First-principles study of the influence of different interfaces and core types on the properties of CdSe/CdS core-shell nanocrystals
- 2015
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 5
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Tidskriftsartikel (refereegranskat)abstract
- With the expanding field of nanoengineering and the production of nanocrystals (NCs) with higher quality and tunable size, having reliable theoretical calculations to complement the experimental results is very important. Here we present such a study of CdSe/CdS core-shell NCs using density functional theory, where we focus on dependence of the properties of these NCs on core types and interfaces between the core and the shell, as well as on the core/shell ratio. We show that the density of states and the absorption indices depend rather weakly on the type of interface and core type. We demonstrate that the HOMO wavefunction is mainly localised in the core of the nanocrystal, depending primarily on the core/shell ratio. On the other hand the LUMO wavefunction spreads more into the shell of the nanocrystal, where its confinement in the core is almost the same in each of the studied structural models. Furthermore, we show that the radiative lifetimes decrease with increasing core sizes due to changes in the dipolar overlap integral of the HOMO and LUMO wavefunctions. In addition, the electron-hole Coulomb interaction energies follow a similar pattern as the localisation of the wavefunctions, with the smaller NCs having higher Coulomb interaction energies.
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| 7. |
- Kocevski, Vancho, 1984-, et al.
(författare)
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Influence of dimensionality and interface type on optical and electronic properties of CdS/ZnS core-shell nanocrystals : a first-principles study
- 2015
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 143:16
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Tidskriftsartikel (refereegranskat)abstract
- Semiconducting nanocrystals (NCs) have become one of the leading materials in a variety of applications, mainly due to their size tunable band gap and high intensity emission. Their photoluminescence properties can be notably improved by capping the nanocrystals with a shell of another semiconductor, making core-shell structures. We focus our study on the CdS/ZnS core-shell nanocrystals, that are closely related to extensively studied CdSe/CdS NCs, albeit exhibiting rather different photoluminescence properties. We employ density functional theory to investigate the changes in the electronic and optical properties of these nanocrystals with size, core/shell ratio and interface structure between the core and the shell. We show that the band gap depends on the size of the NCs and the core/shell ratio. We suggest that the differences in the density of states and absorption are mainly governed by the core/shell ratio. We present that both the LUMO and the HOMO wavefunctions are localised in the core of the NCs, with the distribution of the LUMO wavefunction being more sensitive to the size and the core/shell ratio. We also demonstrate that the Coulomb interaction energies closely follow the behaviour of the localisation of the HOMO and LUMO wavefunctions, and are decreasing with increasing NC size. Furthermore, we investigated the electronic and optical properties of the NCs with different interfaces between the core and the shell, and different core types. We find that the different interfaces and core types have rather small influence on the band gaps and the absorption indices, as well as on the confinement of the HOMO and LUMO wavefunctions. In addition, we compare these results with the previous results for CdSe/CdS NCs, reflecting the different PL properties of these two types of NCs. We argue that the difference in their Coulomb interaction energies is one of the main reasons for their distinct PL properties.
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| 8. |
- Kocevski, Vancho, 1984-, et al.
(författare)
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On the Assessment of Some New Meta-Hybrid and Generalized Gradient Approximation Functionals for Calculations of Anharmonic Vibrational Frequency Shifts in Hydrogen-Bonded Dimers
- 2010
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Ingår i: Journal of Physical Chemistry A. - 1089-5639 .- 1520-5215. ; 114:12, s. 4354-4363
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Tidskriftsartikel (refereegranskat)abstract
- The performance of some recently proposed DFT functionals by Truhlar’s group (mPW1B95, mPWLYP1W,PBELYP1W, and PBE1W [Dahlke, E. E.; Truhlar, D. G. J. Phys. Chem. B 2005, 109, 317. Zhao, Y.; Truhlar,D. G. J. Phys. Chem. A 2004, 108, 6908.]) was tested primarily with respect to computation of anharmonicvibrational frequency shifts upon hydrogen bond formation in small molecular/ionic dimers. Five hydrogenbondedsystems with varying hydrogen bond strengths were considered: methanol-fluorobenzene, phenol-carbonmonoxide in ground neutral (S0) and cationic (D0) electronic states, phenol-acetylene, and phenol-benzene(+).Anharmonic OH stretching frequency shifts were calculated from the computed vibrational potentials forfree and hydrogen-bonded proton-donor molecules. To test the basis set convergence properties, all calculationswere performed with 6-31++G(d,p) and 6-311++G(2df,2pd) basis sets. The mPW1B95 functional was foundto perform remarkably better in comparison to more standard functionals (such as B3LYP, mPW1PW91,PBE1PBE) in the case of neutral dimers. In the case of cationic dimers, however, this is not always the case.With respect to prediction of anharmonic OH stretching frequency shifts upon ionization of free phenol, allDFT levels of theory outperform MP2. Some other aspects of the functional performances with respect tocomputation of interaction and dissociation energies were considered as well.
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| 9. |
- Kocevski, Vancho, 1984-, et al.
(författare)
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Size dependence of the stability, electronic structure, and optical properties of silicon nanocrystals with various surface impurities
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:12
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Tidskriftsartikel (refereegranskat)abstract
- We present a comprehensive, ground-state density functional theory study of the size dependence of the optical and electronic properties and the stability of spherical silicon nanocrystals (NCs) with different impurities on the surface. We vary the size of the NCs from 1.0 to 3.5 nm, considering single-bonded (CH3, F, Cl, OH) and double-bonded (O, S) impurities and bridged oxygen. We show that the density of states (DOS) and absorption indices of the NCs with single-bonded impurities are very similar to each other and the fully hydrogenated NCs, except for the 1.0-nm NCs, where a slight difference is present. In the case of the NCs with double-bonded impurities, the DOS and absorption indices exhibit a significant difference, compared to the fully hydrogenated NCs, for sizes up to 2.5 nm. We argue that this difference arises from the difference in the contribution from the impurity to the states around the gap, which can considerably change the character of the states. We demonstrate that the double-bonded impurities contribute significantly to the states around the gap, compared to the single-bonded impurities, causing changes in the symmetry of these states. This observation was further supported by analyzing the changes of the Fourier transform of the charge densities of the highest occupied and lowest unoccupied eigenstate. We also show that the formation energies of NCs with bridged oxygen and fluorine are the lowest, regardless of the size. Furthermore, we show that high hydrogen concentration can be used to suppress the addition of oxygen and fluorine on the surface of the Si NCs.
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| 10. |
- Kocevski, Vancho, 1984-
(författare)
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Temperature dependence of radiative lifetimes, optical and electronic properties of silicon nanocrystals capped with various organic ligands
- 2018
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Ingår i: Journal of Chemical Physics. - : AMER INST PHYSICS. - 0021-9606 .- 1089-7690. ; 149:5
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Tidskriftsartikel (refereegranskat)abstract
- Despite the known temperature effects on the optical and photoluminescence properties of silicon nanocrystals (Si NCs), most of the density functional theory calculations thus far have been carried out at zero temperature, i.e., fixed atomic positions. We present a study of the effect of finite temperature on the radiative lifetimes and bandgaps of Si NCs capped with six different organic ligands, CH3, C2H5, C2H4Cl, C2H4OH, C2H4SH, and C2H4NH2. In addition, we show the differences in electronic and optical properties, as well as the wavefunctions (WFs) around the bandgap, of the capped Si NCs at zero temperature. We show that the NCs capped with alkyl and C2H4Cl ligands have larger HOMO-LUMO and optical absorption gaps compared to the C2H4NH2, C2H4OH, and C2H4SH capped NCs. We demonstrate that this big difference in both gaps comes from the increased contribution to the states at the top of the valence band from the NH2, OH, and SH groups of the C2H4NH2, C2H4OH, and C2H4SH ligands, respectively. Additionally, we assigned the rather weak dependence of the radiative lifetimes of C2H4NH2 capped NCs on the NC size to the slightly changing symmetry of the highly localized HOMO WF at the NH2 group. Furthermore, we demonstrate that the temperature effect on the radiative lifetimes and bandgaps is larger in alkyl and C2H4Cl capped Si NCs. We indicate that the decrease in radiative lifetime of the CH3 capped NCs with increasing temperature comes from the changing symmetry of the LUMO WF and the increased dipolar overlap between the HOMO and LUMO WFs. Finally, we show that there is a constant decrease in the bandgaps of the Si NCs with increasing size, with the bandgap change of CH3 capped NCs being larger compared to the bandgap change of the C2H4NH2 capped NCs.
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