35 research outputs found
Graphene Electrodes:Universal architecture for 2D electronics
No full textDet atomtynde kulstofmateriale grafen har en ekstraordinære elektroniske egenskaber. Derudover leder det varme bedre end nogen andre materialer, er uhyre stærkt og kemisk stabilt og endda gennemsigtigt. Der er imidlertid en række teknologiske udfordringer der skal overvindes for at grafen kan udnyttes fuldt ud til praktiske anvendelser. PhD afhandlingen omhandler nye måder at fremstille og håndtere de atomtynde film, således at grafenen i processen holdes ren og fri for defekter. Chemical Vapor Deposition bliver brugt til at dyrke grafen ved at opvarme kobber i en atmosfære af metan og hydrogen. Kulstofatomerne danner så sekskantede grafenkrystaller, hvis kvalitet afhænger af kobber-filmens renhed, hvor flad kobberet er samt om kobber er fritstående (folie) eller ligger på en silicium overflade (film). Dyrkning af grafen på folie, som er den konventionelle form for substrat blev sammenlignet med polykrystallinsk film og kobber enkeltkrystaller. En stor del af forskningen omhandler hvordan grafen overføres nænsomt – uden skade på hverken grafen eller kobberoverflade – ved hjælp af en elektrokemisk proces. Den gængse metode hvor vand gennem hydrolyse danner brintbobler mellem grafen og kobber og dermed løsner det, giver generelt skader på grafenen. Ved at sænke potentialet i den elektrokemiske proces, løsnes grafen gennem oxidering af kobberet, hvilket er en langsom men skånsom proces. Denne metode blev brugt til at sammenligne kvaliteten af grafen dyrket på polykrystallinsk kobber film, kommerciel kobber folie samt enkelt-krystallinsk kobber (111). Ved analyser med mikroprober og THz spektroskopi bliver det vist at enkeltkrystallinsk kobber i modsætning til de andre substrater giver anledning til grafen der er kontinuert på både nano- og mikroskala, hvilket vil sige at den elektriske strøm ikke afbrydes af huller, domænegrænser og ridser i grafenen. En særlig flad slags kobberfolie, ”ultrafolie”, blev fremstillet ved at bruge en silicium skive som template, hvilket gav anledning til langt højere renhed og kvalitet af grafen dyrkning end ved både kobber film og folie. Endeligt blev der gennem et 8 måneder besøg ved James Hones forskningsgruppe i Columbia University i New York, udviklet en ny fremstillingsmetode til at opbygge ekstremt rene og fejlfri stakke bestående af forskellige 2D materialer, der udover grafen indbefatter MoS2 og hexagonal Bor Nitrid, og mange andre materialer med vidt forskellige egenskaber. Den foreslåede arktektur til opbygning af 2D hybrid materialer giver enestående mulighed for at studere de fysiske mekanismer bag fremtidens elektronik og elektronikmaterialer. Et kendt problem i grafen forskning er vanskeligheden ved at overføre mønstre til grafen der følger krystalretningerne og har meget lille kantruhed, hvilket skulle give langt bedre kontrol over de elektroniske egenskaber. I projektet blev selv-organiserende litografiske nanometer-skala mønstre dannet i grafen af katalytisk vej, ved at lade sølvpartikler katalysere omdannelsen af grafen-kulstof til carbon monooxid under tilstedeværelsen af oxygen. Disse processer blev filmet og analyseret ved hjælp af et særligt høj-opløsnings transmission elektron mikroskop, med henblik på at svare på om denne metode kan optimeres til at have atomar opløsning.The production of graphene and the other 2D materials is presented in the beginning of this thesis. Micromechanical exfoliation is the best method for obtaining relatively small and top quality samples. The invention of Graphene Finder simplifies the procedure of finding the exfoliated flakes. In this work, largearea films are exclusively obtained by CVD, mostly on copper films and foils. Several forms of copper are used for CVD growth. A new substrate named ultrafoil is invented to overcome the roughness and contamination of commercially available copper foils. The formation of copper silicide in copper thin films is studied and found to be detrimental for the growth of graphene. The modified synthesis of rGO is introduced, as rGO represents a cheap alternative to CVD for large scale production of graphene.The transfer of flakes is performed by several methods, such as with PVA/PMMA support, CAB wedging and the pick-up technique with hBN. Several important improvements of the pick-up technique are introduced. These allowed us to transfer any 2D crystals and patterned graphene flakes with PMMA residues. We also developed the drop-down technique, which is used to release any crystal on the surface of the PPC/PDMS. CVD MoS2 and MoSe2 crystals are transferred from oxide with hBN protection by wedging. Ultra clean suspended crystals are obtained by further adaptation of the pick-up technique.CVD graphene is commonly transferred by etching of the growth substrate or by the bubbling method. Cleaner samples are transferred by combining the drop-down technique and ultrafoil, with hBN flakes protecting the graphene from contacting the polymer support. A new electrochemical transfer method is invented, named ODT. Graphene transferred by ODT shows high coverage compared to other conventional transfer mothods. Another important aspect of the ODT is the preservation of the growth catalyst.THz-TDS is used to generate sheet conductance maps and to characterize the transferred graphene films. Ultrabroad band THz analysis showed a perfect Drude response for graphene grown on copper single crystal and transferred by ODT, while graphene grown on copper foil presented a Drude-Smith response typical of films with extended line defects. M4PP allowed to investigate the continuity of graphene films in the micrometer range. We showed that graphene from Cu single crystal behaved as a perfect 2D conductor, differently from what was previously reported for graphene from Cu foils. An extension of Graphene Finder allowed the generation of high resolution coverage maps that helped characterizing the transferred graphene films.The pick-up transfer method is used to fabricate structures sandwiched in hBN, in which the electrical connection is obtained by one dimensional edge contact. High quality trilayer encapsulated device is presented, with measured mobility more than 5 times higher than any published result. A new architecture for TMDCs based devices is introduced. The crystals are encapsulated in hBN and graphene parts intermediate the (edge) contact between gold and the TMDC. In this way, the MoS2 FET with the highest reported mobility to date has been fabricated. The protection by hBN of CVD graphene grown on ultrafoil allowed to fabricate for the first time encapsulated stacks with CVD graphene.Novel ways of patterning 2D materials are presented. In particular the catalytic etching of graphene by metal nanoparticles is studied. Ag particles in contact with graphene at high temperatures in oxygen are able to form channels aligned along the ZZ direction of graphene. We monitored this phenomenon in-situ with an ETEM. The motion of the particles etching the suspended membrane is discrete, consequence of the interaction with the carbon atoms. DFT calculations supported the hypothesis that it is energetically unfavourable to etch ZZ atoms. The surprisingly strong interaction between the particle and the graphene edge is able to dictate the 3D morphology of the particle. Crystallographic patterning of graphene and hBN is achieved without the catalytic action of metallic particles. Holes are first induced by knock-on damage with high intensity e-beam and enlarged by oxygen at high temperatures
High-quality graphene flakes exfoliated on a flat hydrophobic polymer
No full textWe show that graphene supported on a hydrophobic and flat polymer surface results in flakes with extremely low doping and strain as assessed by their Raman spectroscopic characteristics. We exemplify this technique by micromechanical exfoliation of graphene on flat poly(methylmethacrylate) layers and demonstrate Raman peak intensity ratios I(2D)/I(G) approaching 10, similar to pristine freestanding graphene. We verify that these features are not an artifact of optical interference effects occurring at the substrate: they are similarly observed when varying the substrate thickness and are maintained when the environment of the graphene flake is completely changed, by encapsulating preselected flakes between hexagonal boron nitride layers. The exfoliation of clean, pristine graphene layers directly on flat polymer substrates enables high performance, supported, and non-encapsulated graphene devices for flexible and transparent optoelectronic studies. We additionally show that the access to a clean and supported graphene source leads to high-quality van der Waals heterostructures and devices with reproducible carrier mobilities exceeding 50 000 cm2 V−1 s−1 at room temperature
Silicon nitride membranes nanostencils for patterning and contacting of graphene
No full textLAUREA MAGISTRALENegli ultimi anni, grazie alle sue straordinarie proprietà, quali ad esempio la mobilità dei portatori, nell ordine di centinaia di migliaia di cm2/Vs e la trasparenza alla radiazione visibile, il grafene è stato valutato come uno dei più promettenti candidati per l’implemento di dispositivi di nano-elettronica ed elettronica ultraveloce, quali transistors e sensori.
Inoltre, le proprietà ottiche di questo materiale lo rendono particolarmente adatto all implementazione di dispositivi opto-elettronici, in particolare nel settore delle nanotechnologie e del fotovoltaico, grazie al suo carattere bidimensionale e alle ridotte dimensioni, nell ordine di decine di micrometri.
Prendendo in considerazione i risultati emersi dalle recenti ricerche, questo lavoro vuole essere una guida per il futuro sviluppo di contatti di grafene, utilizzando la tecnologia del Focused Ion Beam (FIB), che offre numerosi vantaggi e potrà essere utilizzata per il milling diretto di flakes, senza l’uso di resist e solventi chimici.
In questa tesi è stato studiato un metodo per la deposizione di contatti metallici su grafene, utilizzando stenciling mask fabbricate sfruttando la tecnica del Focused Ion Beam su membrane si Si3N4, e utilizzando la tecnica del nanostencil.
Per aprire la strada alla produzione di questo tipo di grafene devices, verranno fabricate membrane di Si3N4 direttamente nella cleanroom della Technical University of Denmark, e verrà investigato lo stress intrinseco e indotto dal FIB durante la fabbricazione delle stenciling masks.
Allineando e fissando le membrane a substrati di grafene, sarà possibile trasferire I pattern metallici, e quindi I contatti, sui flakes, per poi studuarne le caratterisciche.Graphene, with his high carrier mobility in the order of hundreds of thousand cm2/Vs has been viewed in recent year as a promising candidate for future high speed electronic devices.
Furthermore, the optical properties of graphene make it particularly suitable for opto electronic integrated devices, especially in nanoscale, since we are dealing with a monolayer material, with thickness of 0.34 nm, which dimensions are typically of some microns.
In this thesis we are investigating the possibility to create grapheme contacts using the focused ion beam (FIB) techniques, meaning that such contacts will be totally contamination free, since no chemical process such as lithography is involved.
The milling of graphene, that is the cut of graphene with focused ion beam, will be done trough thin Si3N4 membranes, of approximately 200nm, in order to reduce the structural defects and redeposition that typically occurs in focused ion beam processes.
Side projects like the fabrications of this membranes and the study of the membranes strenght and stress are included in this work, together with the fabrication of stenciling masks using FIB, in order to deposite this gold contact on graphene flakes and study the electrical properties of this material
Carbon mediated reduction of silicon dioxide and growth of copper silicide particles in uniform width channels
No full textWe show that surface arc-discharge deposited carbon plays a critical intermediary role in the breakdown of thermally grown oxide diffusion barriers of 90 nm on a silicon wafer at 1035°C in an Ar/H2 atmosphere, resulting in the formation of epitaxial copper silicide particles in ≈ 10 μm wide channels, which are aligned with the intersections of the (100) surface of the wafer and the {110} planes on an oxidized silicon wafer, as well as endotaxial copper silicide nanoparticles within the wafer bulk. We apply energy dispersive x-ray spectroscopy, in combination with scanning and transmission electron microscopy of focused ion beam fabricated lammelas and trenches in the structure to elucidate the process of their formation
Graphene and Graphene Metamaterials for Terahertz Absorbers
No full textGraphene, due to the possibility to tune its conductivity, is the promising material for a range of the terahertz (THz) applications, such as tunable reflectors, absorbers, modulators, filters and polarization converters. Subwavelength structuring of graphene in order to form metamaterials allows for even more control over the THz waves. In this poster presentation I will show an elegant way to describe the graphene metamaterials and the design of graphene based absorbers. I will also present our recent experimental results on the graphene absorbers characterization
Graphene Based Terahertz Absorber Designed With Effective Surface Conductivity Approach
No full textYoung field of terahertz (THz) science and technology demands new materials and devices, such as filters, modulators, polarization converters and absorbers. Graphene, a recently discovered single-atom-thick material, provides exciting properties for functional terahertz applications. Graphene is flexible and ultrastrong mechanically, transparent for optical radiation, with high electrical conductivity that can be tuned by electrochemical potential. Structured graphene layers constitute metamaterials that can provide tunable and very unusual electromagnetic properties.In this contribution we present the description of graphene metamaterial properties through the effective surface conductivity. Such description is very convenient, as it simplifies the design of THz devices, and very natural, since surface conductivity can be measured directly in experiment. We show how to extract the effective conductivity and how to use it in optical design. We demonstrate a tunable THz perfect absorber, which consists of continuous graphene various structured graphene metamaterials above a metal mirror. Changing the Fermi level from 0 eV to 0.5 eV allows for drastic changes in absorbance from less than 0.1 to 1 in the working range. We demonstrate the possibility of the absorber bandwidth control with the metamaterial’s unit cell geometry.The results of fabrication and characterization of the THz graphene metamaterials based absorbers will be presented at the conference
Electrochemical method for transferring graphene
No full textThe present application discloses a method for separating a graphene-support layer laminate from a conducting substrate-graphene-support layer laminate, using a gentle, controllable electrochemical method. In this way, substrates which are fragile, expensive or difficult to manufacture can be used - and even re-used - without damage or destruction of the substrate or the graphene
Selective Electroless Silver Deposition on Graphene Edges
No full textWe demonstrate a method of electroless selective silver deposition on graphene edges or between graphene islands without covering the surface of graphene. Modifications of the deposition recipe allow for decoration of graphene edges with silver nanoparticles or filling holes in damaged graphene on silica substrate and thus potentially restoring electric connectivity with minimal influence on the overall graphene electrical and optical properties. The presented technique could find applications in graphene based transparent conductors as well as selective edge functionalization and can be extended to other metals than silver
