Theory of Raman Scattering by Phonons in Germanium Nanostructures

Within the linear response theory, a local bond-polarization model based on the displacement–displacement Green’s function and the Born potential including central and non-central interatomic forces is used to investigate the Raman response and the phonon band structure of Ge nanostructures. In particular, a supercell model is employed, in which along the [001] direction empty-column pores and nanowires are constructed preserving the crystalline Ge atomic structure. An advantage of this model is the interconnection between Ge nanocrystals in porous Ge and then, all the phonon states are delocalized. The results of both porous Ge and nanowires show a shift of the highest-energy Raman peak toward lower frequencies with respect to the Raman response of bulk crystalline Ge. This fact could be related to the confinement of phonons and is in good agreement with the experimental data. Finally, a detailed discussion of the dynamical matrix is given in the appendix section

Computational Modeling of the Size Effects on the Optical Vibrational Modes of H-Terminated Ge Nanostructures

The vibrational dispersion relations of porous germanium (pGe) and germanium nanowires (GeNWs) were calculated using the ab initio density functional perturbation theory with a generalized gradient approximation with norm-conserving pseudopotentials. Both pores and nanowires were modeled using the supercell technique. All of the surface dangling bonds were saturated with hydrogen atoms. To address the difference in the confinement between the pores and the nanowires, we calculated the vibrational density of states of the two materials. The results indicate that there is a slight shift in the highest optical mode of the Ge-Ge vibration interval in all of the nanostructures due to the phonon confinement effects. The GeNWs exhibit a reduced phonon confinement compared with the porous Ge due to the mixed Ge-dihydride vibrational modes around the maximum bulk Ge optical mode of approximately 300 cm−1; however, the general effects of such confinements could still be noticed, such as the shift to lower frequencies of the highest optical mode belonging to the Ge vibrations

Tunable thermal conductivity of ternary alloy semiconductors from first-principles

We compute the thermal conductivity, κ, of five representative III-V ternary alloys - namely In x Ga1 - x As, GaAs1 - x P x , InAs1 - x Sb x , GaAs1 - x N x , and GaP1 - x N x - in the whole range of compositions, and in zincblende and wurtzite crystal phases, using a first-principles approach and solving the phonon Boltzmann transport equation beyond the relaxation time approximation. We discuss the tunability of the thermal conductivity with the composition of the alloy, reporting a steep decrease in the thermal conductivity, followed by a wide plateau and a steep increase common in systems with lattice disorder. We also test the approximation consisting in considering impurities at small values of x as bare mass defects, neglecting their chemical identity, and discuss its validity

Tunable thermal conductivity of ternary alloy semiconductors from first-principles

We compute the thermal conductivity, κ, of five representative III–V ternary alloys—namely InxGa1 − xAs, GaAs1 − xPx, InAs1 − xSbx, GaAs1 − xNx, and GaP1 − xNx—in the whole range of compositions, and in zincblende and wurtzite crystal phases, using a first-principles approach and solving the phonon Boltzmann transport equation beyond the relaxation time approximation. We discuss the tunability of the thermal conductivity with the composition of the alloy, reporting a steep decrease in the thermal conductivity, followed by a wide plateau and a steep increase common in systems with lattice disorder. We also test the approximation consisting in considering impurities at small values of x as bare mass defects, neglecting their chemical identity, and discuss its validity.We acknowledge financial support by the Ministerio de Economía, Industria y Competitividad (MINECO) under Grant FEDER-MAT2017-90024-P and the Severo Ochoa Centres of Excellence Program under Grant CEX2019-000917-S, the Ministerio de Ciencia, Innovación y Universidades under Grant No. RTI2018-097876-B-C21 (MCIU/AEI/FEDER, UE), and by the Generalitat de Catalunya under Grant No. 2017 SGR 1506. MRM is supported by the Ministerio de Educación, Cultura y Deporte through the program of Formación de Profesorado Universitario under Grant No. FPU2016/02565. AM and MCI acknowledge financial support through the multidisciplinary project IPN-SIP 2020-2093 and 2021-0236. FDS acknowledges financial support from scholarships CONACYT-Movilidad and BEIFI-IPN. We thank the Centro de Supercomputación de Galicia (CESGA) for the use of their computational resources. RR thanks Marta De Luca for useful discussions.Peer reviewe

Transition metal-decorated germanene for NO, N2 and O2 sensing: A DFT study

Detecting hazardous and toxic gasses is important to avoid harmful effects on human health and two-dimensional nanostructures have emerged as candidate materials for sensing or scavenging gasses. The chemical interactions between NO, O2, and N2 gas molecules and Cu-, Ag-, and Au-decorated germanene were investigated by using density functional theory simulations, and the potential applications as gas sensors or scavengers were addressed. Except for O2, the studied molecules were physisorbed on pristine germanene, where the most favorable adsorption site is located at the middle of the lattice hexagon, with adsorption energy values ranging from 0.09 eV for the N2 to 0.49 eV for NO adsorbed through the N atom. The results also show that the studied molecules have larger adsorption energies in Cu-, Ag-, and Au-decorated germanene, with energy values of 0.4 eV for the N2 molecule and 1.04 eV for the NO molecule. Therefore, molecule-metal-germanene complexes are more energetically favorable than the molecule-germanene ones and are thus predicted to have an enhanced sensing capability. The larger NO adsorption energies on Ag- (0.8 eV) and Au- (0.87 eV) decorated germanene, in comparison with those of N2 (around 0.1 eV) and O2 (around 0.37 eV), indicate their good selectivity towards NO. To estimate their potential application as NO sensors in gas-insulated switchgear, we calculated the work function and desorption time of the studied molecules adsorbed on Cu-, Ag-, and Au-decorated germanene, obtaining considerable changes in the work function (around 0.5 eV) between the different molecules adsorbed on Cu-decorated germanene, and recovery times of the order of seconds at a temperature of 400 K. The results suggest that metal-germanene complexes are stable in ambient conditions and they are good candidates for sensing and scavenging nitrogen monoxide.This work was partially supported by the multidisciplinary projects IPN-SIP 2020-2093, 2021-0236, 2022-0600 and by UNAM-PAPIIT IN109320. Computations were performed at the supercomputer Miztli of DGTIC-UNAM (Project LANCAD-UNAM-DGTIC-180). RR acknowledges financial support from the Agencia Estatal de Investigación through the Severo Ochoa Centres of Excellence Program under Grant CEX2019-000917-S. A.N.S. and J.E.S. would like to thank CONACYT and BEIFI-IPN for their financial support.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Silicon nanowires as acetone-adsorptive media for diabetes diagnosis

Early detection of diabetes, a worldwide health issue, is key for its successful treatment. Acetone is a marker of diabetes, and efficient, non-invasive detection can be achieved with the use of nanotechnology. In this paper we investigate the effect of acetone adsorption on the electronic properties of silicon nanowires (SiNWs) by means of density functional theory. We considered hydrogenated SiNWs grown along the [1 1 1] bulk Si axis, with group-III impurities (B, Al, Ga), for which both surface substitutional doping and functionalizing schemes are considered. We present an analysis of the adsorption configuration, energetics, and electronic properties of the undoped and doped SiNWs. Upon acetone adsorption, the SiNW without impurities becomes an n-type semiconductor, while most substituted/functionalized cases have their HOMO-LUMO gap tuned, which could be harnessed in optical sensors. Acetone is always chemisorbed, although for the case without impurities, and the Al- and Ga-functionalization schemes, the chemisorption is very weak. These nanostructures could be used for acetone capture and detection, which could lead to applications in the medical treatment of diabetes.This work was partially supported by multidisciplinary projects IPN-SIP 2020-2093, IPN-SIP 2021-0236 and UNAM-PAPIIT IN109320. Computations were done at supercomputer Miztli of DGTIC-UNAM (project LANCAD-UNAM-DGTIC-180 and -381) and at Laboratorio Nacional de Supercómputo del Sureste de México. F.D. and J.E.S. would like to thank CONACYT and BEIFI-IPN for their financial support. R.R. acknowledges financial support by the Ministerio de Economía, Industria y Competitividad (MINECO) [grant number FEDER-MAT2017-90024-P] and the Severo Ochoa Centres of Excellence Program [grant number SEV-2015-0496], and by the Generalitat de Catalunya [grant number 2017 SGR 1506]. The authors wish to acknowledge the contribution of Antonio Torres to the conception and the early development stages of this work.Peer reviewe