Low lying excitations of few electrons in quantum dots

[Excerpt from the Introduction]: This thesis focuses on neutral excitations of few-electrons states in nanofabricatedQDsmeasured by inelastic light scattering. The analysis of the spectra demonstrate that this technique can open a venue in the study of quasi-particles at the nanoscale. A model of electronic states confined in QDs is presented in the first chapter, while the second one provides a survey of the relevant experimental studies in this field. Special attention is paid to the process of inelastic light scattering by introducing a description of the selection rules for the scattering processes by electrons in semiconductor QDs. Chapter 3 describes the nano-fabrication techniques of doped AlGaAs/GaAs QDs and the different setups used for the measurements. Experiments performedwith \u3bc-photoluminescence that demonstrate the confinement and the homogeneity of the fabricated QDs are reported at the end of this chapter. Chapter 4 reports inelastic light scattering spectra fromQDs with many electrons. In these experiments the atomic-like shell structure of the electronic states is demonstrated. The framework used to describe this regime of highly populated QDs is similar to the one used for transport experiments in the Coulomb blockade regime.Observations of tunneling excitations in coupled quantumdots with inelastic light scattering will also be presented. The most innovative results of this work are presented in chapter 5 that focuses on the study of interaction phenomena that take place in QDs in the few electron regime. The correlations that occur among the electrons deeply affect the inelastic light spectra in these systems. Themain outcome regards the case of QDs with four electrons where strong correlation effects have been found in the behavior of spin and charge collective excitations. Light scattering spectra from these dots at zero and finite perpendicularmagnetic fieldswill be presented. The evaluation of the excitation spectra as a function of the magnetic field reveals a ground-state transition between correlated states of the four electrons with different spin configurations

A molecular state of correlated electrons in a quantum dot

Correlation among particles in finite quantum systems leads to complex behaviour and novel states of matter. One remarkable example is predicted to occur in a semiconductor quantum dot (QD) where at vanishing density the Coulomb correlation among electrons rigidly fixes their relative position as that of the nuclei in a molecule. In this limit, the neutral few-body excitations are roto-vibrations, which have either rigid-rotor or relative-motion character. In the weak-correlation regime, on the contrary, the Coriolis force mixes rotational and vibrational motions. Here we report evidence of roto-vibrational modes of an electron molecular state at densities for which electron localization is not yet fully achieved. We probe these collective modes by inelastic light scattering in QDs containing four electrons. Spectra of low-lying excitations associated to changes of the relative-motion wave function -the analogues of the vibration modes of a conventional molecule- do not depend on the rotational state represented by the total angular momentum. Theoretical simulations via the configuration-interaction (CI) method are in agreement with the observed roto-vibrational modes and indicate that such molecular excitations develop at the onset of short-range correlation.Comment: PDF file only; 24 pages, 7 figures, 2 table. Supplementary Information include

Optical control of energy-level structure of few electrons in AlGaAs/GaAs quantum dots

Optical control of the lateral quantum confinement and number of electrons confined in nanofabricated GaAs/AlGaAs quantum dots is achieved by illumination with a weak laser beam that is absorbed in the AlGaAs barrier. Precise tuning of energy-level structure and electron population is demonstrated by monitoring the low-lying transitions of the electrons from the lowest quantum-dot energy shells by resonant inelastic light scattering. These findings open the way to the manipulation of single electrons in these quantum dots without the need of external metallic gates.Comment: To appear in NanoLetter

1.26 μ\mum intersubband transitions in In0.3_{0.3}Ga0.7_{0.7}As/AlAs quantum wells

We observed room-temperature intersubband transitions at 1.26 microns in n-doped type-II In$_{0.3}$Ga$_{0.7}$As/AlAs strained quantum wells. An improved tight-binding model was used to optimize the structure parameters in order to obtain the shortest wavelength intersubband transition ever achieved in a semiconductor system. The corresponding transitions occur between the first confined electronic levels of the well following mid-infrared optical pumping of electrons from the barrier X-valley into the well ground state.Comment: To appear in Applied Physics Letter

TCAD Simulations of High-Aspect-Ratio Nano-biosensor for Label-Free Sensing Application

In this paper, we are presenting simulations of junctionless ion-sensitive field-effect transistor (JL-ISFET) as a pH sensor. Our approach is based on a combination of analytical and numerical methods to reveal the impact of the device geometry and structure on its performance. To have a realistic representation of the fabricated device, further simulations are carried which portray the sensing of surface potential by introducing interface trap charges between the oxide layer and electrolyte. Here, we present our initial steps that belong to a more complex and physically more elaborate simulation framework, which will lead to a better device sensing and fabrication choices of more generic biosensors, in a transition from analytical models to numerical simulations to include effects such as surface roughness and defects in the oxide

Band Depopulation of Graphene Nanoribbons Induced by Chemical Gating with Amino Groups

Altres ajuts: Xunta de Galicia (ED431G/09); Gobierno Vasco (IT1246-19, IT-1255-19); Diputación Foral de Gipuzkoa (RED 2019-096); CERCA Program/Generalitat de Catalunya; Program Interreg V-A España-Francia-Andorra (194/16 TNI)The electronic properties of graphene nanoribbons (GNRs) can be precisely tuned by chemical doping. Here we demonstrate that amino (NH) functional groups attached at the edges of chiral GNRs (chGNRs) can efficiently gate the chGNRs and lead to the valence band (VB) depopulation on a metallic surface. The NH-doped chGNRs are grown by on-surface synthesis on Au(111) using functionalized bianthracene precursors. Scanning tunneling spectroscopy resolves that the NH groups significantly upshift the bands of chGNRs, causing the Fermi level crossing of the VB onset of chGNRs. Through density functional theory simulations we confirm that the hole-doping behavior is due to an upward shift of the bands induced by the edge NH groups

Rational design of a planar junctionless field-effect transistor for sensing biomolecular interactions

In the ElectroMed project, we are interested in screening certain peptide sequences for their ability to selectively interact with antibodies or MHC proteins. This poses a combinatorial challenge that requires a highly multiplexed setup of label-free immunosensors. Label-free FET-based immunosensors are good candidates due to their high multiplexing capability and fast response time. Nanowire-based FET sensors have shown high sensitivity but are unreliable for clinical applications due to drift and gate stability issues. To address this, a label-free immuno-FET architecture based on planar junctionless FET devices is proposed. This geometry can improve the signal-to-noise ratio due to its larger planar structure, which is less prone to defects that cause noise and is better suited to the functionalization of different receptor molecules

Evaluation of 12 GWAS-drawn SNPs as biomarkers of rheumatoid arthritis response to TNF inhibitors. A potential SNP association with response to etanercept

Research in rheumatoid arthritis (RA) is increasingly focused on the discovery of biomarkers that could enable personalized treatments. The genetic biomarkers associated with the response to TNF inhibitors (TNFi) are among the most studied. They include 12 SNPs exhibiting promising results in the three largest genome-wide association studies (GWAS). However, they still require further validation. With this aim, we assessed their association with response to TNFi in a replication study, and a meta-analysis summarizing all nonredundant data. The replication involved 755 patients with RA that were treated for the first time with a biologic drug, which was either infliximab (n = 397), etanercept (n = 155) or adalimumab (n = 203). Their DNA samples were successfully genotyped with a single-base extension multiplex method. Lamentably, none of the 12 SNPs was associated with response to the TNFi in the replication study (p > 0.05). However, a drug-stratified exploratory analysis revealed a significant association of the NUBPL rs2378945 SNP with a poor response to etanercept (B = -0.50, 95% CI = -0.82, -0.17, p = 0.003). In addition, the metaanalysis reinforced the previous association of three SNPs: rs2378945, rs12142623, and rs4651370. In contrast, five of the remaining SNPs were less associated than before, and the other four SNPs were no longer associated with the response to treatment. In summary, our results highlight the complexity of the pharmacogenetics of TNFi in RA showing that it could involve a drug-specific component and clarifying the status of the 12 GWAS-drawn SNPsThis work was supported by the Instituto de Salud Carlos III (ISCIII, Spain) through grants PI14/01651, PI17/01606 and RD16/0012/0014 to AG and PI12/01909 to JJG-R. These grants are partially financed by the European Regional Development Fund of the EU (FEDER