| 1. |
- Akabli, H., et al.
(author)
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Intersubband energies in Al1-yInyN/Ga1-xInxN heterostructures with lattice constant close to aGaN
- 2012
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In: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 52:1, s. 70-77
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Journal article (peer-reviewed)abstract
- We have studied the conduction band profile and the intersubband transition energy, E-12, of Al1-yInyN/Ga1-xInxN quantum well structures. We have considered how material parameters such as non-parabolicity and the uncertainty in the bowing parameter affect E-12 and the corresponding wavelength, lambda(12). The calculations include strain and cover the transition range from telecommunication wavelengths (1.55 mu m) to the mid-infrared (similar to 10 mu m). Our results show that the transition energies of strain-free Al1-yInyN/Ga1-xInxN quantum well structures, which are lattice-matched to GaN (y = 17.7%, x = 0), resulted in wavelengths above similar to 2 mu m. To reach shorter wavelengths, we explored structures with other indium concentrations but maintaining a small mismatch to GaN. For similar to 1% lattice mismatch the wavelength lambda(12) could be reduced to less than 1.55 mu m. The results serve as a starting point for designing and epitaxial growth of photonic intersubband structures.
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| 2. |
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| 3. |
- Bergvall, Anders, 1981, et al.
(author)
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Graphene nanogap for gate-tunable quantum-coherent single-molecule electronics
- 2011
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In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 84:15
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Journal article (peer-reviewed)abstract
- We present atomistic calculations of quantum coherent electron transport through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We predict that three difficult problems in molecular electronics with single molecules can be solved by utilizing graphene contacts: (1) a back gate modulating the Fermi level in the graphene leads facilitates control of the device conductance in a transistor effect with high on-off current ratio; (2) the size mismatch between leads and molecule is avoided, in contrast to the traditional metal contacts; (3) as a consequence, distinct features in charge flow patterns throughout the device are directly detectable by scanning techniques. We show that moderate graphene edge disorder is unimportant for the transistor function.
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| 4. |
- Berland, Kristian, 1983
(author)
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A general solution to the Schrodinger-Poisson equation for a charged hard wall: Application to potential profile of an AlN/GaN barrier structure
- 2011
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In: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 50:4, s. 411-418
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Journal article (peer-reviewed)abstract
- A general, system-independent, formulation of the parabolic Schrodinger-Poisson equation is presented for a charged hard wall in the limit of complete screening by the ground state. It is solved numerically using iteration and asymptotic boundary conditions. The solution gives a simple relation between the band bending and sheet charge density at an interface. Approximative analytical expressions for the potential profile and wave function are developed based on properties of the exact solution. Specific tests of the validity of the assumptions leading to the general solution are made. The assumption of complete screening by the ground state is found be a limitation; however, the general solution provides a fair approximate account of the potential profile when the bulk is doped. The general solution is further used in a simple model for the potential profile of an AlN/GaN barrier structure. The result compares well with the solution of the full Schrodinger-Poisson equation.
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| 5. |
- Berland, Kristian, 1983, et al.
(author)
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A van der Waals density functional study of adenine on graphene: Single-molecular adsorption and overlayer binding
- 2011
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In: Journal of Physics Condensed Matter. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 23, s. 135001-
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Journal article (peer-reviewed)abstract
- The adsorption of an adenine molecule on graphene is studied using a first-principles van der Waals functional, vdW-DF (Dion et al 2004 Phys. Rev. Lett. 92 246401). The cohesive energy of an ordered adenine overlayer is also estimated. For the adsorption of a single molecule, we determine the optimal binding configuration and adsorption energy by translating and rotating the molecule. The adsorption energy for a single molecule of adenine is found to be 711 meV, which is close to the calculated adsorption energy of the similarly sized naphthalene. On the basis of the single-molecular binding configuration, we estimate the cohesive energy of a two-dimensional ordered overlayer. We find a significantly stronger binding energy for the ordered overlayer than for single-molecule adsorption.
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| 6. |
- Berland, Kristian, 1983, et al.
(author)
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Analysis of van der Waals density functional components: Binding and corrugation of benzene and C-60 on boron nitride and graphene
- 2013
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In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 87:20
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Journal article (peer-reviewed)abstract
- The adsorption of benzene and C60 on graphene and boron nitride is studied using density functional theory with the van der Waals density functional (vdW-DF). By comparing these systems we can systematically investigate their adsorption nature and differences between the two functional versions vdW-DF1 and vdW-DF2. The bigger size of the C60 molecule makes it bind stronger to the surface than benzene, yet the interfaces between the molecules and the sheets are similar in nature. The binding separation is more sensitive to the exchange variant used in vdW-DF than to the correlation version. This result is related to the exchange and correlation components of the potential energy curve. We show that a moderate dipole forms for C60 on graphene, unlike for the other adsorption systems. We find that the corrugation (at the atomic scale) is very sensitive to the variant or version of vdW-DF used, in particular, the exchange. Further, we show that this sensitivity arises indirectly through the shift in binding separation caused by changing the vdW-DF variant. Based on our results, we suggest a concerted theory-experiment approach to assess the exchange and correlation contributions to physisorption. Using DFT calculations, the corrugation can be linked to the optimal separation, allowing us to extract the exchange-correlation part of the adsorption energy. Molecules with the same interfaces to the surface, but different geometries, can in turn cast light on the role of van der Waals forces.
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| 7. |
- Berland, Kristian, 1983, et al.
(author)
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Assessment of two hybrid van der Waals density functionals for covalent and non-covalent binding of molecules
- 2017
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In: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 146:23
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Journal article (peer-reviewed)abstract
- Two hybrid van der Waals density functionals (vdW-DFs) are developed using 25% Fock exchange with (i) the consistent-exchange vdW-DF-cx functional [K. Berland and P. Hyldgaard, Phys. Rev. B89, 035412 (2014)] and (ii) with the vdW-DF2 functional [K. Lee et al., Phys. Rev. B 82, 081101 (2010)]. The ability to describe covalent and non-covalent binding properties of molecules is assessed. For properties related to covalent binding, atomization energies (G2-1 set), molecular reaction energies (G2RC set), and ionization energies (G21IP set) are benchmarked against experimental reference values. We find that hybrid-vdW-DF-cx yields results that are rather similar to those of the standard non-empirical hybrid PBE0 [C. Adamo and V. Barone, J. Chem. Phys. 110, 6158 (1999)], with mean average deviations (MADs) of 4.9 and 5.0 kcal/mol for the G2-1 set, respectively. In this comparison, experimental reference values are used, back corrected by wavefunction-based quantum-chemistry calculations of zero-point energies. Hybrid vdW-DF2 follows somewhat different trends, showing on average significantly larger deviations from the reference energies, with a MAD of 14.5 kcal/mol for the G2-1 set. Non-covalent binding properties of molecules are assessed using the S22 benchmark set of non-covalently bonded dimers and the X40 set of dimers of small halogenated molecules, using wavefunction-based quantum chemistry results as references. For the S22 set, hybrid-vdW-DF-cx performs better than standard vdW-DF-cx for the mostly hydrogen-bonded systems, with MAD dropping from 0.6 to 0.3 kcal/mol, but worse for purely dispersion-bonded systems, with MAD increasing from 0.2 to 0.6 kcal/mol. Hybrid-vdW-DF2 offers a slight improvement over standard vdW-DF2. Similar trends are found for the X40 set, with hybrid-vdW-DF-cx performing particularly well for binding energies involving the strongly polar hydrogen halides, but poorly for systems with tiny binding energies. Our study of the X40 set reveals the potential of mixing Fock exchange with vdW-DF, but also highlights shortcomings of the hybrids constructed here. The solid performance of hybrid-vdW-DF-cx for covalent-bonded systems, as well as the strengths and issues uncovered for non-covalently bonded systems, makes this study a good starting point for developing even more accurate hybrid vdW-DFs.
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| 8. |
- Berland, Kristian, 1983
(author)
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Bound by long-range interactions: Molecular crystals and benzene on Cu(111)
- 2009
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Licentiate thesis (other academic/artistic)abstract
- This thesis investigates molecular systems bound by long-range interactions. Westudy molecular crystals held together by van der Waals forces and the physisorption of benzene (Bz) on Cu(111). We also study the self-organization of benzeneon the Cu(111) surface which is influenced both by the (semi) long-range van derWaals forces, and the very-long-ranged indirect electronic interactions mediated bya metallic surface state on the Cu(111) surface. The van der Waals bound systems are investigated from first-principle principlesusing a van der Waals density functional (vdW-DF), [Phys. Rev. Lett. 92, 246401 (2004)]. We examine the character of these interactions, by studying the asymptoticaccount of the van der Waals forces, and compare full vdW-DF with an asymptotic atom-based pair potentials extracted from vdW-DF. The results show that the charac-ter of the binding differs in the two cases, with vdW-DF being significantly enhanced at relevant binding separations. This suggests that the use of asymptotic atom-based pair potentials as an approximation for non-local correlation is not consistent. Formolecular crystals, we study the structure and cohesive energy of hexamine and the platonic hydrocarbons cubane and dodecahedrane. The calculated lattice parame-ters and cohesion energy agree well with experiments. For adsorption of benzene on Cu(111), we obtain a binding energy in good agreement with experiment. We alsostudy diffusion barriers and find that the molecules can move almost freely along a honeycomb web of “corridors” passing between fcc and hcp hollow sites via bridgesites. To account for the surface-state mediated interactions, we use a formal Harris-functional approach to evaluate nonperturbationally the asymptotic form of this in-direct interaction. This description is based on input from DFT calculations of the metallic surface state on Cu(111), in addition to a parameterization of the benzene-induced scattering properties. We combine vdW-DF calculations of benzene dimers and diffusion barriers with the estimates of the surface-state mediated interactionsand find that the more compact (hexagonal) Bz-overlayer phase, with lattice constant ̊a = 6.74 A, is due to direct Bz-Bz vdW attraction, and the stability of the sparser hexagonal Bz phase, a = 10.24 A, is due to surface-state mediated interactions. Thus, we can account well for benzene self-organization on Cu(111). These results illustrate the current state of development of quantum-physical based approaches to systems bound by long-range interactions.
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| 9. |
- Berland, Kristian, 1983
(author)
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Connected by voids: Interactions and screening in sparse matter
- 2012
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Doctoral thesis (other academic/artistic)abstract
- Using theory and computations, we study structural and electronic properties of nanoscale systems where regions of low density are important for response and function. Materials with such properties are called sparse matter. Examples include assemblies of organic molecules and layered structures like graphite. Binding in molecular crystals and physisorption onto surfaces is studied using density-functional theory (DFT) with a van der Waals density functional (vdW-DF) account of exchange-orrelation. We examine binding properties of molecular crystals of high symmetry (hexamine, dodeacahedrane, cubane, C60, graphite) as they allow straightforward analysis of binding nature. We find that vdW-DF describes a non-local attraction that is significantly enhanced at shorter separations compared to the 1/r6 form common in pair-potential descriptions. The binding of adenine, benzene, and C60 on graphene are studied. Several properties of the non-local vdW-DF correlation is elucidated, like sensitivity to geometry, length-scales, and role of exchange companion. Adsorption of H2 and benzene on Cu(111) is studied. We find that benzene affects the surface state and that the corrugation is small enough to allow it to react to surface-state mediated (SSM) interactions. In all cases, the results of vdW-DF calculations compare well to experimental observations.Noble metal surfaces like Cu(111) harbor a metallic Shockley surface state that supports two-dimensional electron gas (2DEG). This surface state is believed to play a crucial role in the self-assembly of molecules. Using DFT, we study this surfacestate and how an external field and physisorbed molecules influence its properties. To aid our study, we developed a method to enhance slab-based calculations of surfacestates. We also investigate the self-assembly of benzene on Cu(111), where we include a simple analytical expression for the SSM interaction in addition to the direct van der Waals interactions. The resulting binding curves have two separate minima that coincide with the periodicity of the measured overlayer phases.Finally, we draw a link to AlN/GaN heterostructures. In analogy to sparse matter, they exhibit wide regions of low conduction electron density. Associated with these depletion regions are inversion layers supporting 2DEGs. This charge redistribution arise from internal fields originating in intrinsic polarization. These structures are studied using a custom-built Schrödinger-Poisson solver. A systematic design strategy to remove the internal fields in the leads is developed to aid the design of devices based on transport perpendicular to the interfaces.
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| 10. |
- Berland, Kristian, 1983, et al.
(author)
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Exchange functional that tests the robustness of the plasmon description of the van der Waals density functional
- 2014
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In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 89:3, s. art. no. 035412-
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Journal article (peer-reviewed)abstract
- Is the plasmon description within the nonlocal correlation of the van der Waals density functional by Dion and coworkers (vdW-DF) robust enough to describe all exchange-correlation components? To address this question, we design an exchange functional based on this plasmon description as well as recent analysis on exchange in the large-s regime. In the regime with reduced gradients s = |del n|/2nk(F)(n) smaller than approximate to 2.5, dominating the nonlocal correlation part of the binding energy, the enhancement factor F-x(s) closely resembles the Langreth-Vosko screened exchange. In the s regime beyond, dominated by exchange, F-x(s) passes smoothly over to the revised Perdew-Wang-86 form. We term the specific exchange functional LV-PW86r, wheras the full van der Waals functional version emphasizing consistent handling of exchange is termed vdW-DF-cx. Our tests indicate that vdW-DF-cx produces accurate separations and binding energies of the S22 data set of molecular dimers as well as accurate lattice constants and bulk moduli of layered materials and tightly bound solids. These results suggest that the plasmon description within vdW-DF gives a good description of both exchange and correlation effects in the low-to-moderate s regime.
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