- Details
- Last Updated: Saturday, 24 December 2022 19:49
Chemical Sensors Operating at the Quantum Frontiers
Goals: Understanding sensing functionalities useful for developing extremely sensitive gas nanosensors that are capable of detecting extremely small traces of highly toxic gases, such as organophosphates, in particular the lethal sarin, and provide the essential elements for designing such sensors.
|
The investigation consists in interactive computer modeling and experimental validation. Multiscale analysis of the signal included detailed knowledge of the thermochemistry of interactions of sarin with the sensor core element, which is a set of order pyramidal SiGe quantum dots (QDs). The coupled dots form a QD array (QDA) and the coupling with nanoscale electrodes bring a series of challenging scientific issues. For instance, the device has quantum properties that are extremely sensitive to symmetry breaking. The sensing functionality is built upon the symmetry breaking that happens as the analyte molecules couple with the QDA |
Design of quantum dot array based sensor. Analyte molecules interact with the surface of a SiGe QD and transduce a quantum based signal. |
|
and get adsorbed by the surface. The adsorption-desorption and surface diffusion of the analyte were analyzed using molecular dynamics (MD) calculations. We found that the sarin molecules, act as rotors that hoover on the surface while spinning around their centers of mass located mid-way between the P and O atom. During the diffusion a gap of 3.2Å on average is maintained between a sarin molecule and the Ge terminated SiGe surface of the QD. We also found an average binding energy of 0.4 eV, demonstrating a strong adsorption. |
Sarin molecule interacting with the surface of a SiGe quantum dot (QD). |
|
Also, the electron energy spectrum was obtained through solving Schrodinger Equation (SE) for the entire system, as a function of the distance of the analyte molecule to the surface of the QD. The solution was obtained by finite element analysis (FEA), where the SE encompensates an effective potential that embodies the electrostatic interactions. These are due to the strained SiGe QDs that have ability to confine charge careers at their tips, and sarin molecules which are polar. Also we obtained the effect of the molecule rotation on the energy spectrum. These analysis led to detailed characterization of the coupling of the analyte (sarin molecules) with SiGe QD surface. The physi-sorption leads to weak quantum coupling which allows for adiabatic interactions. |
Electron confinement at the tip of conical nanodot, in the case of first order eigenvalue. The charge density was calculated by solving SE by FEA and the above described effective potential. |
| The quantum transport was investigated to understand the mechanisms by which electrons are transferred from the analyte to the QD under bias, and from the QDs to the nanoelectrodes. For that purpose non-equilibrium Green’s function (NEGF) formalism was used. The electron density distribution, the tunneling current between QDs and nanoelectrodes, the conductance, and spectral response of the quantum device have been obtained and served as data basis for predictive characterization of the sensing functionalities of our nanoscale sensor operating at the quantum frontiers. |
|
Fabrication of base nanomaterials (layers of SiGe QDs) was done using Ultra High Vacuum Rapid Thermal Chemical Vapor Deposition (UHV-RT-CVD). In the RT-CVD vacuum chamber silane (SiH4) and germane (GeH4) decompose in a series of steps before they form the SiGe precursors, which build-up onto the surface of the silicon substrate to form pyramidal dots. The formation of these SiGe meta-stable precursors depend on the thermodynamic conditions of the gas far from the surface and in the boundary layer. The precursors reduce the energy momentarily, but they themselves decompose when thermodynamic conditions change locally, and the system evolves to even lower energy through heterogenous nucleation process onto the surface. Even though the system evolves towards lowest energy possible state, stable SiGe molecules remain strained. The silicon substrate crystallography and the strain induced driving forces lead to the construction of pyramidal crystals.
|
AFM topography images of as-grown and annealed Si0.83Ge0.17 samples (same scale). Note the QD size is controlled by annealing temperature, as well as the thermal cycling time. |
|
The pyramidal SiGe crystal dots grow gradually, preferentially onto {111} low energy crystallographic planes. Uniform QD layers increase the sensor reliability and lower its cost. The as-grown SiGe QD layers were then annealed using Rapid Thermal Processing. The annealing allowed to control the QD shape and size. A relation QD size- annealing temperature has been established. |
|
|
Non-equilibrium states of SiGe quantum dot arrays (being part of the sensor) and quantum transport of carriers in the Nanosensor Core. We obtained various enlightening results, for instance, the variation of the spectral response, which solves the controversy of invariance of spectra of an artificial atom (quantum dot array) coupled with an analyte. We obtained two variation ranges: small (1 to few meV) for the far field electrostatic interactions, and large (tens to hundreds of meV) for short distance interactions (5 nm). For the latter, the sudden energy variation characterizes the adsorption of sarin by the SiGe QD surface. For the former, the large energy jump is indicative of a significant strain undergone by the sarin molecule during adsorption. The non-uniform electrostatic forces (just like the non-uniform electrostatic potential of the sarin molecule) deform elastically the sarin molecule. |
Calculated energy spectra of Ge QDA (on Si) coupled with a sarin molecule, as a function of the separation distance QDA-molecule. |
|
Characterization of the quantum electronic transport in SiGe-SiO2 core-shell quantum dot connected to two gold electrodes is done by looking at the quantum transport of a QD coupled to two electrodes. A simplified model of such system is analyzed, where only the effect of the core-shell along the quantum dot diameter is considered. |
|
|
|
Using this model and ab-initio DFT calculations combined with Non-Equilibrium Green Function (NEGF) to enable calculation of the electrode self-energy, we obtained: the non-equilibrium electron states, electron density, electron scattering, and electron transmission. These were obtained for various bias conditions. |
|
Subsequently, Current-voltage, Conductance, and Spectral responses were computed.[i] New data of this category, and on the quantum transport in general, will be generated and analyzed in the next phase of this project, to assert the new understanding of stabilizing/delaying the decoherence of the quantum system that we just established this year. We have been able to analyze the scattering of electrons across transverse planes (normal to transport direction) like electron diffraction patterns, an example of such diagram is given. Conventional electron diffraction diagrams are obtained with High |
Transmission-Wave vector spectral diagram of the quadruple heterostructure is shown on the right. |
|
Resolution Electron Transmission Microscopy. This discovered powerful method reveals and depicts in details the electron scattering on the strained atomic bonds of the QD and the interfaces, which contain inhomogeneities of the atomic bonds. When the electrons tunnel through the potential barriers across the heterojunctions, the scattering does occur. The experimental challenging issue related to detection of tunneling current have been overcome by insulating quantum dots individually, between themselves and with the nanoelectrodes. This required the use of extreme processing techniques and highly insulating |
|
|
silicon on insulator (SOI) and germanium on insulator (GOI) substrates. Focused ion beam (FIB) processing of SOI substrate enabled to have QDs on top of pillars of 40nm height separated by15nm gaps. The QDs are coupled to silicon nanoelectrodes, without any physical contacts. The fabrication approach appeared to be very challenging and difficult to improve. Hence, a second approach has been thought-out, where a carbon nanotube (CNT) networks is used. CNT junctions appeared to encompass QDs seamlessly connected to nanowires.
|
HR-TEM image of multi-wall CNT homojunction (CNTj) trapping the analyte (shown with the arrow) at the fork of two connected CNTs. |
   |
|
This study is still on going through both predictive MD and density functional theory with tight binding (DFTB) computer modeling and experimentally. Low concentration of CNTs are dispersed in liquid solution and modified using QDs and other dopants. Initial verification of the sensor has been demonstrated for volatile organic compounds (VOC).
Master Degree thesis
Publications
|