76 research outputs found
Intercalated structures formed by platinum on epitaxial graphene on SiC(0001)
No full textGraphene on SiC intercalated with two-dimensional metal layers, such as Pt, offers a versatile platform for applications in spintronics, catalysis, and beyond. Recent studies have demonstrated that Pt atoms can intercalate at the heterointerface between SiC(0001) and the C-rich
R30°reconstructed surface (hereafter referred to as the buffer layer). However, key aspects such as intercalated phase structure and intercalation mechanisms remain unclear. In this work, we investigate changes in morphology, chemistry, and electronic structure for both buffer layer and monolayer graphene grown on SiC(0001) following Pt deposition and annealing cycles, which eventually led to Pt intercalation at temperatures above 500 °C. Atomic-resolution imaging of the buffer layer reveals a single intercalated Pt layer that removes the periodic corrugation of the buffer layer, arising from partial bonding of C-atoms with Si-atoms of the substrate. In monolayer graphene, the Pt-intercalated regions exhibit a two-level structure: the first level corresponds to a Pt layer intercalated below the buffer layer, while the second level contains a second Pt layer, placed between the former buffer layer and monolayer graphene, giving rise to a
superstructure relative to graphene. Upon intercalation, Pt atoms appear as silicides, indicating a reaction with Si atoms from the substrate. Additionally, charge neutral
-bands corresponding to quasi-free-standing monolayer and bilayer graphene emerge. Analysis of multiple samples, coupled with a temperature-dependent study of the intercalation rate, demonstrates the pivotal role of buffer layer regions in facilitating the Pt intercalation in monolayer graphene. These findings provide valuable insight into Pt intercalation, advancing the potential for applications
Rapid Synthesis of Uniformly Small Nickel Nanoparticles for the Surface Functionalization of Epitaxial Graphene
No full textNickel nanoparticles (Ni NPs) combined with carbon nanomaterials are of significant interest due to their wide range of applications, including catalysis, hydrogen storage, and sensor technologies. However, it is challenging to develop an efficient process to produce small and stable Ni NPs ideal for functionalizing graphene or substrates with complex geometries. For this purpose, a rapid, simple, and cost-effective method is presented for synthesizing uniformly small Ni NPs. The process involves cooling aqueous solutions of Ni(OAc)2 and cetyltrimethylammonium bromide (CTAB) to ≈1 °C, followed by the rapid addition of NaBH4. Crucial parameters, such as temperature and stirring rate, are precisely controlled to ensure uniform particle growth, with the reaction completing in just a few minutes. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) characterizations reveal spherical NPs with an average diameter of ≈11 nm and a narrow size distribution. Additionally, epitaxial graphene (EG) samples are functionalized with the synthesized NPs and their arrangement on the surface and their stability upon thermal annealing are investigated. X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) measurements demonstrate the degradation of CTAB, along with the recovery of Ni(0) under mild conditions (below 350 °C), with the NPs maintaining structural stability up to approximately 550 °C
Ultrafast hot carrier transfer in WS2/graphene large area heterostructures
No full textCharge transfer processes in two-dimensional van der Waals heterostructures enable upconversion of low energy photons and efficient charge carriers extraction. Here we use broadband ultrafast optical spectroscopy to track charge transfer dynamics in large-area 2D heterostructures made of epitaxial single-layer tungsten disulfide (WS2) grown by chemical vapour deposition on graphene. Selective carrier photoexcitation in graphene, with tunable near-infrared photon energies as low as 0.8 eV (i.e. lower than half of the optical bandgap of WS2), results in an almost instantaneous bleaching of the WS2 excitonic peaks in the visible range, due to the interlayer charge transfer process. We find that the charge transfer signal is strongly non-linear with the pump fluence and it becomes progressively more linear at increasing pump photon energies, while the interlayer photoinjection rate is constant in energy, reflecting the spectrally flat absorbance of graphene. We ascribe the interlayer charge transfer to a fast transfer of hot carriers, photogenerated in graphene, to the semiconducting layer. The measured sub-20-fs hot-carrier transfer sets the ultimate timescale for this process. Besides their fundamental interest, our results are technologically relevant because, given the capability of large-area deterministic growth of the heterostructure, they open up promising paths for novel 2D photodetectors, also potentially scalable to industrial platforms
Großflächiges epitaktisches Graphen auf SiC(0001): von der Entkopplung bis zum Maßschneidern der Grenzfläche
No full textThe fascinating electronic properties of graphene gave the possibility to investigate quantum electrodynamics phenomena in a solid-state playground, setting it therefore as an
extremely interesting material which was theoretically studied for six
decades prior to its experimental isolation.
The epitaxial graphene (EG) on silicon carbide (SiC) was the first realization of
this one-atom thick material and it still appears to be the solution of highest
potentiality for a concrete application of graphene in commercial electronics. As
a purely two-dimensional material, the influence of the substrate in the determination
of the graphene electronic properties is of crucial importance. The first
EG layer on SiC(0001) (buffer layer) strongly interacts with the substrate, suppressing
the development of the graphene π-bands. The possibility to intercalate
foreign atomic species at the interface to relieve the buffer layer from the interaction
with the substrate has been a breakthrough in the graphene research. In
particular, it has been observed that different intercalants modify the graphene
electronic properties in different ways. In addition to this, the graphene electronic
properties are also affected by adsorbants and by the ordering of those as well as
of the intercalated elements. The electronic properties of graphene, like carrier
concentration, extrinsic spin-orbit coupling or band gap opening can be therefore
selectively modified (or induced) by choosing the correct element or compound to
deposit onto or intercalate under the EG on SiC.
The present thesis follows the mainstream of that research branch presenting
experimental data on the electronic properties of large-area epitaxial graphene
on SiC(0001), modified by the intercalation of foreign atomic species at the heterointerface
between the graphene and the substrate. Prior to the investigation
of the sample properties, details about the description of the growth methodologies
and parameters will be provided. Several surface analysis techniques have
been used to characterize the properties of the material, like angle-resolved photoelectron
spectroscopy (ARPES), low-energy electron microscopy (LEEM) and
diffraction (LEED), atomic force microscopy (AFM) and core-level photoelectron
spectroscopy (CLPES). A background about the general main graphene electronic
properties as well as the working principles of the listed experimental techniques
will be provided, together with a brief overview of the SiC properties. The core of
the thesis will be focused in first place on the possibility to relieve the buffer layer
graphene from the interaction with the substrate, rendering in this way the graphene
decoupled. In particular, the quasiparticle dynamics in quasi-free standing
monolayer graphene will be discussed and an estimation of the electron-phonon
coupling constant will be provided. The growth of high-quality epitaxial bilayer
graphene (BLG) will be described and discussed and high-resolution ARPES data
of large-area quasi-free standing trilayer graphene (QFTLG), obtained for the first
time on SiC by the H-intercalation of BLG, will be shown. A relevant weight will
be given to the intercalation of foreign atomic species, deposited from solid-state
sources. In those cases, element- and concentration-dependent electronic properties
are induced on the graphene by the intercalants. In particular, it will
be shown that an intercalated ML of Cu atoms orders itself with a (13 x 13)
periodicity with respect to the graphene and that such a graphene superlattice
exhibits strong renormalization effects of its low-energy excitations dispersion. In
particular, novel Dirac-like states centered in the non-equivalent corners of the
superlattice mini Brillouin zone are observed. The intercalation of Sb atoms under
the EG is achieved by means of a novel, modified deposition/intercalation
process. The Sb-intercalated graphene is n-type doped and exhibits an unusual
thermal and chemical stability, together with peculiar morphological properties
related to the spatial distribution and the chemical state of the Sb at the interface.
Eventually, one-dimensional anisotropic etching stimulated by catalytic nanoparticles
and self-assembly of 4H-SiC(0001) vicinal surfaces will be introduced as the
routes pursued to carve nanoribbons starting from EG on SiC(0001).Die elektronischen Eigenschaften von Graphen sind einzigartig im Forschungsgebiet der Festkörperphysik und zeichnen Graphen als außergewöhnliches Material aus. Die Energiedispersion der niederenergetischen Anregungen im Graphen verläuft in der Tat linear (konisch) mit dem Impulsvektor (im Bereich der Ecken der Graphen-Brillouinzone), Graphen zeichnet sich daher durch eine erstaunliche Analogie zu quanten-elektrodynamischen (QED) Systemen aus. Im Laufe der Jahre wurde das Auftreten einer Reihe von Effekten theoretisch vorhergesagt, wie Supraleitfähigkeit oder die Erzeugung neuer Dirac Kegel, wenn die elektronischen Eigenschaften des Graphen durch externe Störungen maßgeschneidert werden können. Vor allem in den letzten Jahren sind nun viele experimentelle Ansätze zu verzeichnen, die Bandstruktur von Graphen zu modifizieren, um die erwünschten Effekte zu erzeugen. Tatsächlich wurde demonstriert, dass epitaktisches Graphen (EG) auf Siliziumkarbid (SiC) – modifiziert durch verschiedene adsorbierte oder interkalierte Elemente – bestimmte Effekte zeigen kann. In diesem Zusammenhang widmet sich diese Doktorarbeit der Veränderung der Eigenschaften von EG auf SiC, als Antwort auf die Interkalation verschiedener fremder Atomsorten. Sie gibt eine detaillierte Beschreibung der resultierenden Strukturen und ihrer elektronischen Zustände. Die intensive und umfassende Charakterisierung der Graphenproben wird durch eine Kombination verschiedener sich ergänzender Oberflächenanalysemethoden ermöglicht. Vor allem die Verwendung von winkelaufgelöster Photoelektronenspektroskopie (ARPES) erlaubt die Untersuchung der elektronischen Struktur von Graphen und die Bestimmung von Oberflächenzuständen in der Nähe des Fermi-Niveaus, seien es die ursprünglichen Grapheneigenschaften oder neue Zustände induziert durch interkalierte Atome. Die chemische Analyse der Proben wurde durch Röntgen-Photoelektronen-Spektroskopie (XPS) und durch Röntgen-Photoelektronenemissions-Mikroskopie (XPEEM) mit Hilfe von Synchrotron-Lichtquellen gemacht. Durch die hohe Intensität und Kohärenz, sowie die variable Photonenenergie war die Synchrotronstrahlung fundamental für Qualität und Auflösung der gemessenen Spektren. Die kristalline Struktur, sowie die Oberflächenmorphologie wurden durch Beugung und Mikroskopie niederenergetischer Elektronen (LEED, LEEM) untersucht. Detaillierte Informationen über die lokale Rauigkeit und Morphologie der Oberfläche wurden mittels Rasterkraftmikrokopie (AFM) erhalten.
Diese Arbeit bietet eine allgemeine Einführung in die grundlegenden elektronischen Eigenschaften von Graphen, einen Überblick über die Eigenschaften von Siliziumkarbid sowie eine Beschreibung der verwendeten experimentellen Methoden. Des Weiteren befasst sie sich mit der Entkopplung Graphens von seinem Substrat durch die Interkalation verschiedener Fremdatome und die einhergehende Wechselwirkung dieser Fremdatome mit dem Graphen.
In diesem Zusammenhang wird die Dynamik der Quasiteilchen in wasserstoffinterkaliertem Monolagengraphen diskutiert und ein Ansatz zur Beschreibung der Elektron-Phonon-Wechselwirkung gegeben. Das Wachstum epitaktischem Bilagengraphens (BLG) von hoher Qualität wird beschreibt und diskutiert. Es werden hochaufgelöste ARPES Messungen von wasserstoffinterkaliertem BLG gezeigt, welche zum ersten Mal auf SiC(0001) aufgenommen werden konnten. Eine hohe Relevanz wird darüber hinaus der Interkalation verschiedener Fremdatome zugesprochen, welche aus Festkörperquellen aufgedampft und durch thermische Behandlung zur Interkalation gebracht werden. In diesen Fällen sind die elektronischen Eigenschaften des Graphens abhängig vom verwendeten Material und der Konzentration des interkalierten Materials. Insbesondere, Kupfer bildet bei der Interkalation einer Monolage unter einer Pufferlage eine (13x13)-Überstruktur in Bezug auf das Graphen aus. Dieses Übergitter hat dabei starke Renormalisierungseffekten der niederenergetischen elektronischen Anregungen im Graphen zur Folge. Im Speziellen, entstehen nahe den K-Punkten des Graphens neue Dirac-Fermionen-Zustände, welche in zusätzlichen kegelförmigen Zweigen der Dispersionsrelation resultieren, den sogenannten „Mini Dirac Cones“. Die Interkalation von Antimon unter epitaktisches Graphen wird durch eine neue, modifizierte Depositions- und Interkalationsmethode erzielt. Das mit Antimon interkalierte Graphen ist n-dotiert und zeigt eine außerordentliche thermische und chemische Stabilität sowie eine außergewöhnliche Morphologie, welche aus der räumlichen Verteilung der Antimonatome in unterschiedlichen chemischen Zuständen folgt. Zuletzt wird das eindimensionale anisotrope Ätzen von Graphen durch katalytische Nanopartikel zusammen mit der Selbstorganisierung von vizinalen 4H-SiC(0001)-Oberflächen diskutiert. Diese Methoden werden als Möglichkeit herangezogen, um Nanostreifen von epitaktischem Graphen auf SiC (0001) herzustellen
Novel Structures of Gallenene Intercalated in Epitaxial Graphene
No full textThe creation of atomically thin layers of non-exfoliable materials remains a crucial challenge, requiring the development of innovative techniques. Here, confinement epitaxy is exploited to realize 2D gallium (gallenene) via intercalation in epitaxial graphene grown on silicon carbide. Both fabrication and characterization are conducted under ultra-high vacuum conditions, unlike previous works on intercalated gallenene, to avoid gallium oxidation. Gallium is deposited on the graphene substrate via molecular beam epitaxy, and the intercalation is achieved by thermal treatments, leading to a homogeneous intercalation on almost the entire surface of the samples. Novel superstructures, including a striped and a hexagonal moiré pattern, are discovered and investigated via STM and LEED measurements. These structures arise from the interaction of gallenene with graphene and the silicon carbide substrate. The coexistence of different gallenene phases, including b010-gallenene and the unprecedented 2D-Ga(III) phase, is identified. This work sheds new light on the formation of 2D gallium and identifies a new tailored procedure for fabricating different phases of confined Ga, offering a platform for investigating the exotic electronic and optical properties of gallenene
Adhesion and friction patterns of CVD-grown WS2 monolayer flakes induced by vacancy-rich defect domains
No full textA comprehensive understanding of the relationship between defects concentration, optical properties and mechanical behavior of two-dimensional transition metal dichalcogenides (TMDs) is crucial for their integration as active components in micro- and nanomechanical devices. In this study we characterize the nanoscale contact adhesion and friction of WS2 flakes grown via chemical vapor deposition. We identify two domains named α and β with distinct mechanical properties, which are not apparent in morphological differences but mirror spatial variations of the optoelectronic properties. The α-domains exhibit high photoluminescence (PL) emission, strong Raman response, higher contact adhesion and lower friction, closely resembling the response of pristine WS2 flakes prepared by mechanical exfoliation. Conversely, the β-domains display very low PL emission, weak Raman response with blueshifted fingerprint peaks, lower adhesion and up to six-fold higher friction. Based on experimental evidence and general arguments, we attribute the mechanical heterogeneity between the α- and β-domains to the differentiated densities of sulfur and tungsten atomic vacancies, which are known to selectively populate the two domains. Our results indicate that the tungsten vacancies in the β-domains not only mediate non-radiative recombination processes but also drive a prominent friction enhancement, either by increasing the amplitude and disorder of the WS2 potential energy surface or by impacting the stress distribution within the growing flakes. These findings help identify the type of defects and mechanisms that most significantly affect the properties of TMD monolayer flakes prepared by scalable production routes
Towards AI-driven autonomous growth of 2D materials based on a graphene case study
No full textThe scalable synthesis of two-dimensional (2D) materials remains a key challenge for their integration into solid-state technology. While exfoliation techniques have driven much of the scientific progress, they are impractical for large-scale applications. Advances in artificial intelligence (AI) now offer new strategies for materials synthesis. This study explores the use of an artificial neural network (ANN) trained via evolutionary methods to optimize graphene growth. The ANN autonomously refines a time-dependent synthesis protocol without prior knowledge of effective recipes. The evaluation is based on Raman spectroscopy, where outcomes resembling monolayer graphene receive higher scores. This feedback mechanism enables iterative improvements in synthesis conditions, progressively enhancing sample quality. By integrating AI-driven optimization into material synthesis, this work contributes to the development of scalable approaches for 2D materials, demonstrating the potential of machine learning in guiding experimental processes. (Figure presented.
Black Phosphorus nType Doping by Cu: A Microscopic Surface Investigation
No full textWe study surface charge transfer doping of exfoliated black phosphorus (bP) flakes by copper using scanning tunneling microscopy (STM) and spectroscopy (STS) at room temperature. The tunneling spectra reveal a gap in correspondence of Cu islands, which is tentatively attributed to Coulomb blockade phenomena. Moreover, using line spectroscopic measurements across small copper islands, we exploit the potential of the local investigation, showing that the n-type doping effect of copper on bP is short-ranged. These experimental results are substantiated by first-principles simulations, which quantify the role of cluster size for an effective n-type doping of bP and show an electronic decoupling of the topmost bP layer from the underlying layers driven by the copper cluster, consistent with the Coulomb blockade interpretation. Our results provide novel understanding—difficult to retrieve by transport measurements—of the doping of bP by copper, which appears promising for the implementation of ultrasharp p-n junctions in bP
Built-in Bernal gap in large-angle-twisted monolayer-bilayer graphene
No full textAtomically thin materials offer multiple opportunities for layer-by-layer control of their electronic properties. While monolayer graphene (MLG) is a zero-gap system, Bernal-stacked bilayer graphene (BLG) acquires a finite band gap when the symmetry between the layers’ potential energy is broken, usually, via a displacement electric field applied in double-gate devices. Here, we introduce a twistronic stack comprising both MLG and BLG, synthesized via chemical vapor deposition, showing a Bernal gap in the absence of external fields. Although a large (~30°) twist angle decouples the MLG and BLG electronic bands near Fermi level, proximity-induced energy shifts in the outermost layers result in a built-in asymmetry, which requires a displacement field of 0.14 V/nm to be compensated. The latter corresponds to a ~10 meV intrinsic BLG gap, a value confirmed by our thermal-activation measurements. The present results highlight the role of structural asymmetry and encapsulating environment, expanding the engineering toolbox for monolithically-grown graphene multilayers
Azobenzene-based optoelectronic transistors for neurohybrid building blocks
No full textAbstract Exploiting the light–matter interplay to realize advanced light responsive multimodal platforms is an emerging strategy to engineer bioinspired systems such as optoelectronic synaptic devices. However, existing neuroinspired optoelectronic devices rely on complex processing of hybrid materials which often do not exhibit the required features for biological interfacing such as biocompatibility and low Young’s modulus. Recently, organic photoelectrochemical transistors (OPECTs) have paved the way towards multimodal devices that can better couple to biological systems benefiting from the characteristics of conjugated polymers. Neurohybrid OPECTs can be designed to optimally interface neuronal systems while resembling typical plasticity-driven processes to create more sophisticated integrated architectures between neuron and neuromorphic ends. Here, an innovative photo-switchable PEDOT:PSS was synthesized and successfully integrated into an OPECT. The OPECT device uses an azobenzene-based organic neuro-hybrid building block to mimic the retina’s structure exhibiting the capability to emulate visual pathways. Moreover, dually operating the device with opto- and electrical functions, a light-dependent conditioning and extinction processes were achieved faithful mimicking synaptic neural functions such as short- and long-term plasticity
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