63 research outputs found
Intrinsic defect engineering of CVD grown monolayer MoS for tuneable functional nanodevices
Defects in atomically thin materials can drive new functionalities and expand
applications to multifunctional systems that are monolithically integrated. An
ability to control formation of defects during the synthesis process is an
important capability to create practical deployment opportunities. Molybdenum
disulfide (MoS), a two-dimensional (2D) semiconducting material harbors
intrinsic defects that can be harnessed to achieve tuneable electronic,
optoelectronic, and electrochemical devices. However, achieving precise control
over defect formation within monolayer MoS, while maintaining the
structural integrity of the crystals remains a notable challenge. Here, we
present a one-step, in-situ defect engineering approach for monolayer MoS
using a pressure dependent chemical vapour deposition (CVD) process. Monolayer
MoS grown in low-pressure CVD conditions (LP-MoS) produces sulfur
vacancy (Vs) induced defect rich crystals primarily attributed to the kinetics
of the growth conditions. Conversely, atmospheric pressure CVD grown MoS
(AP-MoS) passivates these Vs defects with oxygen. This disparity in defect
profiles profoundly impacts crucial functional properties and device
performance. AP-MoS shows a drastically enhanced photoluminescence, which
is significantly quenched in LP-MoS attributed to in-gap electron donor
states induced by the Vs defects. However, the n-doping induced by the Vs
defects in LP-MoS generates enhanced photoresponsivity and detectivity in
our fabricated photodetectors compared to the AP-MoS based devices.
Defect-rich LP-MoS outperforms AP-MoS as channel layers of field-effect
transistors (FETs), as well as electrocatalytic material for hydrogen evolution
reaction (HER). This work presents a single-step CVD approach for in-situ
defect engineering in monolayer MoS and presents a pathway to control
defects in other monolayer material systems.Comment: 29 pages, 5 figure
Oxygen-deficient photostable Cu2O for enhanced visible light photocatalytic activity
Oxygen vacancies in inorganic semiconductors play an important role in reducing electron-hole recombination, which may have important implications in photocatalysis. Cuprous oxide (Cu2O), a visible light active p-type semiconductor, is a promising photocatalyst. However, the synthesis of photostable Cu2O enriched with oxygen defects remains a challenge. We report a simple method for the gram-scale synthesis of highly photostable Cu2O nanoparticles by the hydrolysis of a Cu(i)-triethylamine [Cu(i)-TEA] complex at low temperature. The oxygen vacancies in these Cu2O nanoparticles led to a significant increase in the lifetimes of photogenerated charge carriers upon excitation with visible light. This, in combination with a suitable energy band structure, allowed Cu2O nanoparticles to exhibit outstanding photoactivity in visible light through the generation of electron-mediated hydroxyl (OH) radicals. This study highlights the significance of oxygen defects in enhancing the photocatalytic performance of promising semiconductor photocatalysts.V. B. thanks the Australian Research Council (ARC) for a
Future Fellowship (FT140101285) and funding support
through an ARC Discovery (DP170103477). ARC is also
acknowledged for DECRA Fellowships to E. D. G.
(DE170100164) and J. v. E. (DE150100427) and a Future
Fellowship to N. C. (FT1401000834). M. S. acknowledges RMIT
University for an Australian Postgraduate Award (APA). A. E. K.,
E. D. G., P. R. and R. R. acknowledge RMIT University for Vice
Chancellor Fellowships. V. B. recognizes the generous support
of the Ian Potter Foundation toward establishing an Ian Potter
NanoBioSensing Facility at RMIT University. The authors
acknowledge the support from the RMIT Microscopy and
Microanalysis Facility (RMMF) for technical assistance and
providing access to characterization facilities. This work was
also supported by the ARC Centre of Excellence for Nanoscale
BioPhotonics (CE140100003)
Silicon as a ubiquitous contaminant in graphene derivatives with significant impact on device performance
Silicon-based contaminants are ubiquitous in natural graphite, and they are thus expected to be present in exfoliated graphene. Here, the authors show that such impurities play a non-negligible role in graphene-based devices, and use high-purity parent graphite to boost the performance of graphene sensors and supercapacitor microelectrodes
Noble Metal / Metal Oxide nanocomposite thin films for optical gas sensors
In the last decades, the research field known as nanotechnology has been deeply investigated since it helps to understand the properties of the materials, and provides a useful tool to design materials with tailored properties, that can be exploited for many applications across the whole field of science. Nanomaterials exhibit distinctive size-dependent properties, and a high surface to volume ratio, extremely useful in applications like sensing and catalysis.
In this doctoral project, different combinations of noble metals and transition metal oxides have been used to prepare inorganic thin films to be used as reducing gases sensors through an optical interface: while the semiconductive metal oxide is usually responsible for the detection mechanism, metal nanoparticles play the role of optical probes, enhancing the optical response, and/or catalysts, improving the sensor performances. The main work presented here was focused on the synthesis of these nanocomposite materials through different strategies, according to the desired quality of the final material, the easiness of the procedure, the control on key aspects like size and shape of the particles, their size distribution, the crystallinity of the different components, the porosity.
In the first part, noble metal (Au, Ag, Pt) ions have been embedded inside oxide matrixes by means of sol-gel or impregnation processes, and reduced to metal nanoparticles through high temperature annealing, which is necessary also to promote the oxides crystallization: remarkable gas sensing properties have been observed for NiTiO3-TiO2-Au films for hydrogen sulfide detection, with extremely good sensitivity and selectivity towards interfering gases like CO and H2. The experimental results suggest a catalytic oxidation of H2S to sulfur oxides promoted by NiTiO3 crystals, while Au nanoparticles are not involved directly in the reaction mechanism, but act as probes providing an easily detectable optical signal. Quite good sensing properties for CO and hydrogen detection have been presented for other nanocrystalline thin films like SiO2-NiO-Ag prepared combining sol-gel and impregnation processes, sol-gel ZnO-NiO-Au nanocomposites, and microstructured WO3-Au-Pt films synthesized with the sputtering technique and a subsequent impregnation process.
The second part is based on the colloidal synthesis of metal (Au, Pt, Au@Pt core@shell) and oxide (TiO2, ZnO pure and doped with transition metal ions) nanoparticles with desired size and distribution: purification and concentration protocols have been developed and the final colloidal solutions have been directly used for films deposition, obtaining nanocrystalline coatings at low temperatures. TiO2-based films show good sensitivity for CO and H2, with a detection threshold of about 2 ppm, quite remarkable considering that films are only 40-60 nm thick. These materials were also able to detect ethanol vapors at room temperature. Moreover samples containing both Au and Pt NPs are able to reversibly detect hydrogen at room temperature, thanks to the synergetic effect occurring between the optical properties of Au and the catalytic properties of Pt. ZnO-based samples have been tested as CO sensors with a detection limit down to 1-2 ppm, and a relationship between type of dopant (Ni, Co, Mn) and response intensity has been presented.
The third part is focused on the deposition of Au nanoparticles layers on properly functionalized substrates, and their subsequent coating with sol-gel films: when Au nanoparticles are in close contact with each other, a coupling of the plasmon frequencies is found to occur, and this effect can be used to enhance sensing, SERS and catalytic performances. Au nanoparticles layers covered with NiO or TiO2 films showed promising gas sensing properties for CO and hydrogen detection at high temperatures, and for ethanol sensing at low temperatures. More complex structures composed of an Au nanoparticles layer sandwiched between two different oxide layers (NiO, TiO2, ZnO) are also prepared, trying to enhance the selectivity towards interfering gases by providing two different noble metal / metal oxide interfaces.Negli ultimi decenni, il campo delle nanotecnologie è stato largamente studiato, poiché tramite esso si è in grado di comprendere le proprietà dei materiali, ed esso stesso fornisce un mezzo per progettare materiali aventi le proprietà desiderate, che possono essere utilizzati in diverse applicazioni nell’intero campo della scienza. I nanomateriali presentano interessanti proprietà dipendenti dalla dimensione delle particelle, e inoltre il rapporto superficie-volume in questi materiali è estremamente alto, il che li rende utili per applicazioni in sensoristica e catalisi.
In questo progetto di dottorato, diverse combinazioni di metalli nobili e ossidi di metalli di transizione sono state sfruttate per preparare film sottili inorganici, utilizzati come sensori ottici di gas riducenti: solitamente l’ossido semiconduttivo è responsabile per il meccanismo di rilevazione, mentre le nanoparticelle metalliche agiscono da sonde ottiche, aumentando la sensibilità , e/o da catalizzatori, migliorando le prestazioni del sensore. Il principale lavoro presentato in questa tesi è stato focalizzato sulla sintesi di questi materiali attraverso diverse strategie, a seconda della qualità desiderata per il materiale finale, della semplicità operativa, del controllo su parametri chiave come forma e dimensione delle particelle, la loro distribuzione dimensionale, la cristallinità dei diversi costituenti, la porosità .
Nella prima parte, ioni di metalli nobili (Ag, Au, Pt) sono stati inseriti all’interno di matrici di ossidi attraverso sintesi sol-gel o processi di impregnazione, e successivamente ridotti a particelle metalliche attraverso trattamenti termici ad alta temperatura, che sono necessari anche per la cristallizzazione degli ossidi: i sistemi NiTiO3-TiO2-Au hanno dimostrato notevoli proprietà sensoristiche nella rilevazione di acido solfidrico, con elevata sensibilità e selettività nei confronti di gas interferenti quali H2 e CO. I risultati sperimentali suggeriscono un effetto dei cristalli di NiTiO3 nel promuovere l’ossidazione catalitica dell’H2S a ossidi di zolfo, mentre le nanoparticelle di oro non sono coinvolte direttamente nella reazione, ma agiscono come sonde ottiche, producendo un segnale ottico facilmente rilevabile.
Discreti risultati per la rilevazione di CO e idrogeno sono stati presentati per altri film sottili nanocristallini, come SiO2-NiO-Ag, preparati combinando la tecnica sol-gel e il processo di impregnazione, film sol-gel a base di una matrice di ZnO e NiO contenenti nanoparticelle di Au, e film microstrutturati di WO3 contenenti nanoparticelle di Au e Pt sintetizzati combinando sputtering e impregnazione.
La seconda parte di questa tesi è basata sulla sintesi colloidale di nanoparticelle di metalli (Au, Pt, Au@Pt core@shell) e di ossidi (TiO2, ZnO puro e drogato con ioni di metalli di transizione), aventi la desiderata dimensione e distribuzione dimensionale: protocolli di purificazione e concentrazione sono stati sviluppati, e le soluzioni ottenute sono state direttamente utilizzate per la deposizione di film sottili, ottenendo così rivestimenti nanocristallini a bassa temperatura. I film a base di TiO2 hanno mostrato buona sensibilità per idrogeno e CO, con un limite di rilevazione di circa 2 ppm, notevole se considerato che i film sono spessi solo 40-60 nm. Inoltre questi materiali si sono dimostrati capaci di rilevare vapori di etanolo a temperatura ambiente. Infine, campioni contenenti nanoparticelle di oro e platino sono in grado di rilevare idrogeno a temperatura ambiente, grazie all’effetto sinergico che avviene tra le proprietà ottiche dell’oro e quelle catalitiche del platino. I film a base di ZnO sono stati testati come sensori di CO, dimostrando una soglia di rilevazione di circa 1-2 ppm, e una relazione fra il tipo di dopante utilizzato (Ni, Co, Mn) e l’intensità della risposta è stata presentata.
La terza parte è focalizzata sulla deposizione di strati di nanoparticelle di oro su substrati opportunamente funzionalizzati, e il loro successivo ricoprimento con film sol-gel: quando le particelle di oro sono molto vicine le une alle altre, le risonanze plasmoniche si accoppiano, e questo effetto può essere sfruttato per migliorare le prestazioni in ambiti quali sensoristica, SERS e catalisi. Strati di particelle di Au ricoperti da film di NiO o TiO2 hanno mostrato promettenti proprietà per la rilevazione di CO e idrogeno ad alte temperature, e di vapori di etanolo a basse temperature. Inoltre, strutture più complesse a base di uno strato di particelle di oro immobilizzato fra due film di ossidi diversi (NiO, TiO2, ZnO) sono state preparate, con lo scopo di migliorare la selettività verso gas interferenti, fornendo due diverse interfacce metallo/ossido
Gold nanoparticles to boost the gas sensing performance of porous sol\u2013gel thin films
In this paper we review our research work of
the last few years on the synthesis and the gas sensing
properties of nanocomposite thin films of sensitive materials
with a large specific surface area, which consist of
porous matrices containing functional nanocrystals of
metal oxides and gold. The film porosity provides a path
for the gas molecules to reach the active reaction sites on
the nanoparticles surface undergoing chemical reactions
which nature depends on the nature of the active material.
The introduction of Au nanoparticles affects the reactions
mechanism improving the sensing process, moreover the
Au Surface Plasmon Resonance peak can be used for the
realization of selective optical gas sensor. Two different
synthetic approaches will be described, each of them
characterized by a peculiar control of the final materials
morphology, structure and micro-structure
TiO2-NiO nanocomposite thin films with Au nanoparticles for optical H2S detection
Nanocomposite thin films com -
posed of Au nanoparticles dispersed inside
a TiO2-NiO mixed oxide matrix are
synthesized with the sol-gel method and
characterized structurally and morphologically.
Optical gas sensing tests have
been perform ed under H2S, CO, H2 and
propane exposure, showing high sensitivity
and selectivity towards hydrogen sulfide
detection. A possible reaction
mechanism is also reported
SiO2 mesoporous thin films containing Ag and NiO nanoparticles synthesized combining sol\u2013gel and impregnation techniques
SiO2 sol\u2013gel mesoporous thin films containing NiO nanoparticles have been synthesized and successively impregnated with Ag ions. A subsequent thermal treatment induces the precipitation of Ag nanoparticles and the formation of Ag/NiO interfaces. Morphological, structural and optical characterizations are used to analyze the matrix evolution with thermal annealing and to investigate the Ag+ impregnation procedure and the Ag nanocrystals precipitation. NiO nanoparticles form upon annealing at 800 \u25e6C while the SiO2 matrix is still highly porous due to the organic template removal. This porosity is useful for the impregnation process that is composed by a first pores functionalization with mercapto moieties and a second impregnation with Ag precursor solution followed by the thermal reduction of Ag ions. Optical gas sensing tests for CO and H2 detection are presented as one of the possible applications for these noble metal\u2013metal oxides nanocomposites
Spectroscopic ellipsometry analyses of thin films in different environments: An innovative \u201creverse side\u201d approach allowing multi angle measurements
An innovative ellipsometer sample holder has been designed and tested in order to measure thin films optical properties under different environments and so infer the porosity through effective medium approximation models. Compared to commercial cells that require a fixed angle of incidence or a cell with a cylindrical geometry, we present a simple cell in which the sample is mounted in \u2018\u2018reverse side\u2019\u2019, allowing multiple angle analyses without the need for cell windows. Standard ellipsometry measurements are compared to the \u2018\u2018reverse side\u2019\u2019 approach in order to confirm the feasibility of this new procedure, obtaining the same refractive index dispersion curves in both cases. Then different samples have been tested in \u2018\u2018reverse side\u2019\u2019 under different environments to measure porosity. The multiangle approach has been found useful to improve the fitting of the experimental data by reducing both the fitting error and the correlation between parameters
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