Over the last decades, artificial nanostructures grown with atomic-scale precision have become the cutting edge of materials physics. One of their salient features is the posibility of restricting the motion of the electrons to two, one or zero dimensions.
Fast response photogating in monolayer MoS2 phototransistors
D. Vaquero, V. Clericò, J. Salvador-Sánchez, E. Díaz, F. Domínguez-Adame, L. Chico, Y. M. Meziani, E. Diez and J. Quereda
Nanoscale 13, 16156 (2021)
We investigate the photoresponse of a fully h-BN encapsulated monolayer MoS2 phototransistor. In contrast with previous understanding, we identify a rapidly-responding photogating effect mechanism that becomes the dominant contribution to photoresponse under high-frequency light modulation. Using a Hornbeck–Haynes model for the photocarrier dynamics, we fit the illumination power dependence of this photogating effect and estimate the energy level of the involved traps. The resulting energies are compatible with shallow traps in MoS2 caused by the presence of sulfur vacancies.
Spin filtering induced by a magnetic insulator stripe on graphene
C. H. Fuentevilla, J. D. Lejarreta, F. Domínguez-Adame and E. Diez
New Journal of Physics 23, 053029 (2021)
Proximity exchange interaction between graphene electrons and nearby magnetic insulators paves the way to create spin-polarised currents for spintronics applications. Different ferro- and ferrimagnetic insulators, such as europium chalcogenides, yttrium iron garnet and cobalt ferrite, have been proposed in the literature to induce magnetic correlations in graphene. We theoretically study electronic transport properties of graphene in close proximity to a strip of a magnetic insulator, when the system is connected to nonmagnetic source and drain leads. To this end, we describe graphene electrons by means of an effective Hamiltonian whose model parameters are extracted from first-principle calculations. We compare the spin-polarization of the electron current calculated for a number of different magnetic insulators, aiming at elucidating the effects of the various model parameters on the efficiency of the device. In particular, we demonstrate that the polarization of the electric current across the device can be tuned by the source–drain voltage. We conclude that the heterostructures based on europium chalcogenides are ideal candidates to achieve high polarisation at low temperature.
Excitons, trions and Rydberg states in monolayer MoS2 revealed by ...
D. Vaquero, V. Clericò, J. Salvador-Sánchez, A. Martín-Ramos, F. Domínguez-Adame, Y. M. Meziani, E. Diez and J. Quereda
Communications Physics 33, 194 (2020)
Exciton physics in two-dimensional semiconductors are typically studied by photoluminescence spectroscopy. However, this technique does not allow for direct observation of non-radiating excitonic transitions. Here, we use low-temperature photocurrent spectroscopy as an alternative technique to investigate excitonic transitions in a high-quality monolayer MoS2 phototransistor. The resulting spectra presents excitonic peaks with linewidths as low as 8 meV. We identify spectral features corresponding to the ground states of neutral excitons (XA1s and XB1s) and charged trions (TA and TB) as well as up to eight additional spectral lines at energies above the XB1s transition, which we attribute to the Rydberg series of excited states of XA and XB. The intensities of the spectral features can be tuned by the gate and drain-source voltages. Using an effective-mass theory for excitons in two-dimensional systems we are able to accurately fit the measured spectral lines and unambiguously associate them with their corresponding Rydberg states.
Tuning the thermoelectric reponse of silicene nanoribbons with vacancies
C. Núñez, M. Saiz-Bretín, P. A. Orellana, L. Rosales and F. Domínguez-Adame
Journal of Physics: Condensed Matter 32, 275301 (2020)
In this work, we present a thorough study of the thermoelectric properties of silicene nanoribbons in the presence of a random distribution of atomic vacancies. By using a linear approach within the Landauer formalism, we calculate phonon and electron thermal conductances, the electric conductance, the Seebeck coefficient and the figure of merit of the nanoribbons. We found a sizable reduction of the phonon thermal conductance as a function of the vacancy concentration over a wide range of temperature. At the same time, the electric properties are not severely deteriorated, leading to an overall remarkable thermoelectric efficiency. We conclude that the incorporation of vacancies paves the way for designing better and more efficient nanoscale thermoelectric devices.
Impact of device geometry on electron and phonon transport in graphene nanorings
M. Saiz-Bretín, L. Medrano Sandonas, R. Gutierrez, G. Cuniberti and F. Domínguez-Adame
Physical Review B 99, 165428 (2019)
Recent progress in nanostructuring of materials opens up possibilities to achieve more efficient thermoelectric devices. Nanofilms, nanowires, and nanorings may show increased phonon scattering while keeping good electron transport, two of the basic ingredients for designing more efficient thermoelectric systems. Here we argue that graphene nanorings attached to two leads meet these two requirements. Using a density-functional parametrized tight-binding method combined with Green's function technique, we show that the lattice thermal conductance is largely reduced as compared to that of graphene nanoribbons. At the same time, numerical calculations based on the quantum transmission boundary method, combined with an effective transfer matrix method, predict that the electric properties are not considerably deteriorated, leading to an overall remarkable thermoelectric efficiency. We conclude that graphene nanorings can be regarded as promising candidates for nanoscale thermoelectric devices.
Nanowires: A route to efficient thermoelectric devices
F. Domínguez-Adame, M. Martín-González, D. Sánchez and A. Cantarero
Physica E 113, 213 (2019)
Miniaturization of electronic devices aims at manufacturing ever smaller products, from mesoscopic to nanoscopic sizes. This trend is challenging because the increased levels of dissipated power demands a better understanding of heat transport in small volumes. A significant amount of the consumed energy in electronics is transformed into heat and dissipated to the environment. Thermoelectric materials offer the possibility to harness dissipated energy and make devices less energy-demanding. Heat-to-electricity conversion requires materials with a strongly suppressed thermal conductivity but still high electronic conduction. Nanowires can meet nicely these two requirements because enhanced phonon scattering at the surface and defects reduces the lattice thermal conductivity while electric conductivity is not deteriorated, leading to an overall remarkable thermoelectric efficiency. Therefore, nanowires are regarded as a promising route to achieving valuable thermoelectric materials at the nanoscale. In this paper, we present an overview of key experimental and theoretical results concerning the thermoelectric properties of nanowires. The focus of this review is put on the physical mechanisms by which the efficiency of nanowires can be improved. Phonon scattering at surfaces and interfaces, enhancement of the power factor by quantum effects and topological protection of electron states to prevent the degradation of electrical conductivity in nanowires are thoroughly discussed.
Topologically protected states in δ-doped junctions with band inversion
A. Díaz-Fernández, N. del Valle, E. Díaz and F. Domínguez-Adame
Physical Review B98, 085424 (2021)
A topological boundary can be formed at the interface between a trivial and a topological insulator. The difference in the topological index across the junction leads to robust gapless surface states. Optical studies of these states are scarce in the literature, the reason being the difficulty in isolating their response from that of the bulk. In this work, we propose to deposit a δ layer of donor impurities in close proximity to a topological boundary to help in detecting gapless surface states. As we will show, gapless surface states are robust against this perturbation and they enhance intraband optical transitions as measured by the oscillator strength. These results help us to understand the interplay of surface and bulk states in topological insulators.
Quantized electron transport through graphene nanoconstrictions
V. Clericò, J. A. Delgado-Notario, M. Saiz-Bretín, C. Hernández Fuentevilla, A. V. Malyshev, J. D. Lejarreta, E. Diez and F. Domínguez-Adame
Physica Status Solidi 215, 1701065 (2018)
The quantization of Dirac fermions in lithographically defined graphene nanoconstrictions is studied. Quantized conductance is observed in single nanoconstrictions fabricated on top of a thin hexamethyldisilazane layer over a Si/SiO2 wafer. This nanofabrication method allows to obtain well defined edges in the nanoconstrictions, thus reducing the effects of edge roughness on the conductance. The occurrence of ballistic transport is proved and several size quantization plateaus are identified in the conductance at low temperature. Experimental data and numerical simulations show good agreement, demonstrating that the smoothening of the plateaus is not related to edge roughness but to quantum interference effects.