The necessity of sustainability in energy production and the continuous increasing of global warming, which leads to tremendous consequences, are among the most complicated challenges facedby humanity along its history. Reduction of the energy wastes anda strong energetic efficiency improvement are the most relevant solutions proposed, since nearly the 60 % of the energy generated around the world is wasted as heat. The possibility to recover even a small amount of this wasted energy could lead to a significant decrease of CO2 emission. Thermoelectric devices can actively contribute to this cause sincethey allow to generate electrical power even with small temperature gradients and without moving parts. Their efficiency is described by the figure of merit zT. Therefore, an ideal thermoelectric material should have, at the same time,good electrical properties combined to a low thermal conductivity ,a difficult challenge considering that, normally, a good electrical conductor is also a good thermal conductor. However, property modification at nanoscale opened a new pathway in thermoelectric materials research. The work of this PhD thesis is focused on the nanostructuration of a non-toxic, earth-abundant material such as Silicon. Due to the high thermal conductivity, bulk silicon is not suitable for thermoelectric application. Anyway, nanostructuration offers efficient and innovative ways to lower silicon thermal conductivity and to open novel opportunities to its usage as thermoelectric material. In the first part, the mechanism of Silver-assisted Chemical etching (SaCE), a one-step method chosen for the production of silicon NW will be presented. Particularly, the results of anextended analysis of the interplay among doping level and type of silicon, nanowire morphology and the parameters controlling thechemistry of SaCE will be shown. SaCE occurs at the outer substrate surface as a result of Si extrusion by sinking self-propelled Ag particles which causes Si flakes to be exposed at the outer solution-substrate. Here, the etching actually occurs through either 2- or 4-electron electrochemical oxidation of Si. NW surface is found to be either porous (potholed) or crystalline depending on the predominant electrochemical process. The prevalence of either 2- or4-electron processes is controlled by the material resistivity andtherefore by the voltage sensed by silicon. Two-electron processes occur at low voltages for conductive, heavily doped Si,and causes the formation of superficially potholed NWs. Four-electron processes occur for weakly doped Si and lead to fully crystalline NWs.Secondly, the production, by means of SaCE, and the characterization of a recently introduced category of material, the so-called Nanophononic Metamaterial (NPM), will be presented. This material is composed by an array of silicon nanopillars on top of a silicon thin film. The hybridization of the locally-resonant phonon modes introduced by the NWs with membrane phonon modes leads to a thermal conductivity reduction. NPM demonstrates to retain electrical and thermal conductivity of the wafer from which it is etched. Preliminary thermal measurements showed a thermal conductivity reduction of 2/3 with respect of bulk silicon. In the third part, the characterization of heavily doped Si NWs arrays, produced by SaCE, will be presented. This kind of arrays shows very low thermal conductivity (around 2 W/ (m K)) and a Seebeck coefficient comparable with that of heavily doped bulk silicon. Anyway, due to the presence of the substrate (very thick if compared with NWs length), it is complicated to have a precise measurement of NW resistivity. To overcome this issue, a new structure exclusively made of NWs and free from any substrate contribution will be presented.

Il mio progetto di tesi prevede la preparazione di materiali a base di nanofili di silicio, sfruttando una via sintetica in soluzione: il Metal-assisted Chemical Etching (MaCE). La tecnica prevede l’immersione del substrato monoscristallino <100> di Silicio in una soluzione di acido fluoridrico, contenente una sorgente di ioni Ag+ (AgNO3). Il processo consiste sostanzialmente nell’ossidazione localizzata del Silicio, catalizzata dagli ioni Ag+; l’ossido di Silicio così formato viene successivamente disciolto dal HF presente in soluzione, permettendo la formazione di nanofili per etching chimico. Nonostante il MaCE sia una tecnica diffusamente utilizzata a livello sperimentale, l’effettivo meccanismo del processo è ancora fortemente dibattuto in letteratura. Grazie al mio periodo a Marsiglia, ho potuto caratterizzare a fondo dal punto di vista morfologico i vari nanofili ottenuti da substrati a diverse concentrazioni di drogante, diverse specie droganti. E’ stata, inoltre, variata sistematicamente la temperatura di attacco, nonché la concentrazione di Ag+ all’interno della soluzione. I risultati ottenuti grazie ad una avanzata analisi morfologica con SEM (Scanning Electron Microscopy) e TEM (Transmission Electron Microscopy) hanno permesso di aprire una riflessione e avanzare teoria su diversi aspetti dell’etching, dal trasferimento elettronico alla localizzazione dell’attacco. La versatilità del MaCE permette la sintesi di un metamateriale, introdotto nel 2014 da Davis et al, costituito da una membrana di Silicio sulla quale è posto un array di nanofili di Silicio e definito “Nanophononic Metamaterial (NPM)”. L’interazione tra i modi fononici introdotti dai nanofili all’interno del film e i modi del film stesso porterebbe il NPM ad una conducibilità termica del 48%, rispetto a quella del corrispettivo film sottile senza nanopillars, grazie una ibridizzazione delle curve di dispersione fononica e la comparsa di modi fononici piatti e localmente risonanti. Inoltre, visto che il trasporto elettronico avviene nella membrana che rimane priva di difetti o inclusioni, le proprietà elettroniche del NPM risultano conservate, rendendolo ideale per applicazioni termoelettriche vista la bassa conducibilità termica risultante. NPM con diversi spessori di membrana sono stati prodotti partendo da un wafer Double-Side-Polished di 200 micron di spessore, sul quale sono stati prodotti i nanofili tramite MaCE, su entrambe le facce. Scegliendo la lunghezza dei nanofili è stato possibile regolare lo spessore della membrana residua. Le caratterizzazioni elettriche e termoelettriche hanno dimostrato come il comparto elettronico del NPM sia mantenuto. La caratterizzazione termica di una membrana con spessore di 62 micron ha ottenuto una conducibilità termica pari al 36% di quella del Silicio bulk. Questo materiale, quindi, permette di disaccoppiare la conducibilità elettrica (regolata dalle caratteristiche della membrana) dalla conducibilità termica (controllata dalla presenza dei nanofili), rendendolo ideale per applicazioni termoelettriche.

(2021). Thermoelectric nanostructured silicon obtained by Metal-assisted Chemical Etching. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2021).

Thermoelectric nanostructured silicon obtained by Metal-assisted Chemical Etching

MAGAGNA, STEFANO
2021

Abstract

The necessity of sustainability in energy production and the continuous increasing of global warming, which leads to tremendous consequences, are among the most complicated challenges facedby humanity along its history. Reduction of the energy wastes anda strong energetic efficiency improvement are the most relevant solutions proposed, since nearly the 60 % of the energy generated around the world is wasted as heat. The possibility to recover even a small amount of this wasted energy could lead to a significant decrease of CO2 emission. Thermoelectric devices can actively contribute to this cause sincethey allow to generate electrical power even with small temperature gradients and without moving parts. Their efficiency is described by the figure of merit zT. Therefore, an ideal thermoelectric material should have, at the same time,good electrical properties combined to a low thermal conductivity ,a difficult challenge considering that, normally, a good electrical conductor is also a good thermal conductor. However, property modification at nanoscale opened a new pathway in thermoelectric materials research. The work of this PhD thesis is focused on the nanostructuration of a non-toxic, earth-abundant material such as Silicon. Due to the high thermal conductivity, bulk silicon is not suitable for thermoelectric application. Anyway, nanostructuration offers efficient and innovative ways to lower silicon thermal conductivity and to open novel opportunities to its usage as thermoelectric material. In the first part, the mechanism of Silver-assisted Chemical etching (SaCE), a one-step method chosen for the production of silicon NW will be presented. Particularly, the results of anextended analysis of the interplay among doping level and type of silicon, nanowire morphology and the parameters controlling thechemistry of SaCE will be shown. SaCE occurs at the outer substrate surface as a result of Si extrusion by sinking self-propelled Ag particles which causes Si flakes to be exposed at the outer solution-substrate. Here, the etching actually occurs through either 2- or 4-electron electrochemical oxidation of Si. NW surface is found to be either porous (potholed) or crystalline depending on the predominant electrochemical process. The prevalence of either 2- or4-electron processes is controlled by the material resistivity andtherefore by the voltage sensed by silicon. Two-electron processes occur at low voltages for conductive, heavily doped Si,and causes the formation of superficially potholed NWs. Four-electron processes occur for weakly doped Si and lead to fully crystalline NWs.Secondly, the production, by means of SaCE, and the characterization of a recently introduced category of material, the so-called Nanophononic Metamaterial (NPM), will be presented. This material is composed by an array of silicon nanopillars on top of a silicon thin film. The hybridization of the locally-resonant phonon modes introduced by the NWs with membrane phonon modes leads to a thermal conductivity reduction. NPM demonstrates to retain electrical and thermal conductivity of the wafer from which it is etched. Preliminary thermal measurements showed a thermal conductivity reduction of 2/3 with respect of bulk silicon. In the third part, the characterization of heavily doped Si NWs arrays, produced by SaCE, will be presented. This kind of arrays shows very low thermal conductivity (around 2 W/ (m K)) and a Seebeck coefficient comparable with that of heavily doped bulk silicon. Anyway, due to the presence of the substrate (very thick if compared with NWs length), it is complicated to have a precise measurement of NW resistivity. To overcome this issue, a new structure exclusively made of NWs and free from any substrate contribution will be presented.
NARDUCCI, DARIO
ALFONSO, CLAUDE
Silicon; Thermoelectric; Nanowires; Etching; Seebeck
Silicio; Termoelettricità; Nanofili
CHIM/02 - CHIMICA FISICA
English
14-apr-2021
SCIENZA E NANOTECNOLOGIA DEI MATERIALI
33
2019/2020
Aix-Marseille University - Aix-Marseille Université
open
(2021). Thermoelectric nanostructured silicon obtained by Metal-assisted Chemical Etching. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2021).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/312087
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