Ανάπτυξη Γραφενίου και Συνδυασμός με Διηλεκτρικά Υλικά για Νέες Ηλεκτρονικές Διατάξεις

Graphene is a two-dimensional material with very promising prospects for applications on electronic devices of new generation. In this work we have optimized the process to grow single layer graphene on copper substrates by chemical vapor deposition. Large areas of single layer graphene with low defect densities were certified by Raman spectroscopy. Although Raman spectroscopy is widely used to study the number of graphene layers when the layers are AB stacked, the method has limitations when it deals with other stacking orders. We demonstrated that ARPES measurements can be used to determine the number of layers of non-AΒ stacked graphene grown on single crystal Cu(111). Thus, we found that we had three rotated graphene layers with a relative angle of ~4° between them. Another approximation to the single layer graphene was tried. From a sample with AB stacked multi-layer graphene, the number of layers were reduced by etching the sample at hydrogen atmosphere while using mild temperatures during the annealing. However, we did not achieve our principal goal to reduce the sample to single layer graphene. Instead we managed to reduce the sample to AB-stacked few-layers graphene, which is also interesting since it is not easy to grow large areas of AB stacked few-layers graphene by chemical vapor deposition. It is worth mentioning that the process did not introduce noticeable defects on the lattice. The whole process was certified by Raman spectroscopy and XPS. We achieved to encapsulate graphene in MIS device gates. The transfer method was optimized, as well as the Al2O3 deposition, to avoid graphene defects. C-V curves were measured on capacitors with and without graphene embed in order to analyze the effect of graphene in the structure. The results show a large enhancement of the total capacitance of up to 35% in the accumulation, above the geometrical capacitance. This was related to the negative quantum capacitance contribution of graphene by a theoretical model of the device. Finally, the stability of graphene on different two-dimensional substrates such as hAlN/Ag111, h-BN/Ni111, HfSe2/Ni111 and MoSe2/TaSe2 was studied by density functional theory. Also, the study of the electronic properties of the structures mentioned before shows that dielectrics with a larger band gap (h-AlN, h-BN and MoSe2) do not affect the electronic properties of graphene while the dielectric with smaller band gap (HfSe2) clearly affect the graphene LDOS. From the point of view of metals, it is shown that Ag(111) alters more significantly the LDOS of graphene than Ni(111) or TaSe2. Additionally, DFT method was used to analyze ARPES measurements of different twodimensional transition metal dichalcogenides such as TaSe2, ZrSe2 and HfSe2.Το γραφένιο είναι ένα δισδιάστατο υλικό με πολλά υποσχόμενες προοπτικές για εφαρμογές σε ηλεκτρονικές διατάξεις νέας γενιάς. Στο πλαίσιο αυτής της διδακτορικής διατριβής, βελτιστοποιήθηκε η διαδικασία ανάπτυξης μονοατομικού στρώματος γραφενίου σε υποστρώματα χαλκού, μέσω της μεθόδου Χημικής Εναπόθεσης από Φάση Ατμών (Chemical Vapor Deposition, CVD). Για τον έλεγχο των αποτελεσμάτων, μεγάλες επιφάνειες μονοατομικού γραφενίου με μικρή πυκνότητα ατελειών χαρακτηρίστηκαν μέσω της φασματοσκοπίας Raman. Παρά το γεγονός ότι η φασματοσκοπία Raman χρησιμοποιείται ευρέως για την ανίχνευση του αριθμού των στρωμάτων AB stacked γραφενίου, η μέθεοδος δεν είναι επαρκής όταν χρησιμοποιείται σε άλλες στρωματικές δομές. Ως απάντηση σε αυτό το πρόβλημα, αποδείχτηκε ότι η μέθοδος ARPES μπορεί να χρησιμοποιηθεί για να προσδιοριστεί ο αριθμός των στρωμάτων του Νon-AΒ stacked γραφενίου επί μονοκρυσταλλικου Cu(111). Το συμπέρασμα είναι ότι αυτές οι δομές αποτελούνται από τρία στρώματα γραφενίου, περιστραμμένα μεταξύ τους, ανά δύο, κατά ~4°. Επιχειρήθηκε επιπλέον μία ακόμα προσέγγιση για την ανάπτυξη μονοατομικού στρώματος γραφενίου. Από ένα δείγμα AB stacked πολυστρωματικού γραφενίου, ο αριθμός των στρωμάτων μειώθηκε με διάβρωση του δείγματος σε ατμόσφαιρα υδρογόνου και με χρήση ήπιας θερμοκρασίας. Ωστόσο δεν καταφέραμε να επιτύχουμε τον αρχικό μας στόχο και να παράξουμε μονοατομικό στρώμα γραφενίου. Αντί αυτού καταφέραμε όμως να μειώσουμε το δείγμα σε AB-stacked ολιγοστρωματικό γραφένιο, κάτι το οποίο έχει επίσης ενδιαφέρον, δεδομένου ότι μέχρι τώρα ήταν δύσκολο να επιτευχθεί η ανάπτυξη μεγάλων επιφανειών AB-stacked γραφενίου μέσω της CVD. Αξίζει επίσης να σημειωθεί ότι η παραπάνω διαδικασία δεν προκάλεσε εμφανή ελαττώματα στο πλέγμα. Τα πειραματικά αποτελέσματα επιβεβαιώθηκαν μέσω της φασματοσκοπίας Raman και της XPS. Σε συνέχεια των προηγούμενων, καταφέραμε να ενσωματώσουμε γραφένιο σε πύλες MIS. Τόσο η διαδικασία μεταφοράς του γραφενίου στις πύλες όσο και η εναπόθεση του Al2O3 βελτιστοποιήθηκαν προκειμένου να αποφευχθούν τα ελαττώματα στο γραφένιο. Στη συνέχεια μετρήθηκαν οι καμπύλες C-V των πυκνωτών, μέ και χωρίς γραφένιο, για να προσδιοριστεί η επίδραση του γραφενίου στη λειτουργία της συνολικής δομής. Τα αποτελέσματα έδειξαν μία μεγάλη αύξηση έως και 35% της συνολικής πυκνότητας στη συσσώρευση, πάνω από τη γεωμετρική πυκνότητα. Αυτή αποδόθηκε στην συμμετοχή της αρνητικής κβαντικής πυκνότητας του γραφενίου, μετά από θεωρητική αναπαράσταση του μοντέλου της εν λόγω διάταξης. Τέλος, εξετάστηκε η σταθερότητα του γραφενίου επάνω σε διαφορετικά δισδιάστατα υποστρώματα όπως τα h-AlN/Ag111, h-BN/Ni111, HfSe2/Ni111 και MoSe2/TaSe2, μέσω της θεωρίας συναρτησιακού της πυκνότητας (DFT). Επιπλέον, από τη μελέτη των xii ηλεκτρονιακών ιδιοτήτων των δομών που αναφέρθηκαν παραπάνω, προέκυψε ότι τα διελεκτρικά υλικά με μεγαλύτερο ενεργειακό χάσμα (h-AlN, h-BN and MoSe2) δεν επηρεάζουν τις ηλεκτρονιακές ιδιότητες του γραφενίου, ενώ αυτά με μικρότερο ενεργειακό χάσμα (HfSe2) σαφώς επηρεάζουν περισσότερο σημαντικά την τοπική πυκνότητα καταστάσεων (LDOS) του γραφενίου. Ως προς τα μέταλλα, αποδείχθηκε ότι ο Ag(111) διαφοροποιεί πιο έντονα τα LDOS του γραφενίου σε σχέση με το Ni(111) ή το TaSe2. Τέλος επισημαίνεται ότι η μέθοδος DFT χρησιμοποιήθηκε για να αναλυθούν οι μετρήσεις ARPES των διαφορετικών δισδιάστατων μεταβατικών μετάλλικών διχαλκογενίδων όπως τα TaSe2, ZrSe2 και HfSe2

Surface-Enhanced Raman Spectroscopy of Graphene Integrated in Plasmonic Silicon Platforms with Three-Dimensional Nanotopography

Integrating graphene with plasmonic nanostructures results in multifunctional hybrid systems with enhanced performance for numerous applications. In this work, we take advantage of the remarkable mechanical properties of graphene to combine it with scalable three-dimensional (3D) plasmonic nanostructured silicon substrates, which enhance the interaction of graphene with electromagnetic radiation. Large areas of femtosecond laser-structured arrays of silicon nanopillars, decorated with gold nanoparticles, are integrated with graphene, which conforms to the substrate nanotopography. We obtain Raman spectra at 488, 514, 633, and 785 nm excitation wavelengths, spanning the entire visible range. For all excitation wavelengths, the Raman signal of graphene is enhanced by 2–3 orders of magnitude, similarly to the highest enhancements measured to date, concerning surface-enhanced Raman spectroscopy of graphene on plasmonic substrates. Moreover, in contrast to traditional deposition and lithographic methods, the fabrication method employed here relies on single-step, maskless, cost-effective, rapid laser processing of silicon in water, amenable to large-scale fabrication. Finite-difference time-domain simulations elucidate the advantages of the 3D topography of the substrate. Conformation of graphene to Au-decorated silicon nanopillars enables graphene to sample near fields from an increased number of nanoparticles. Due to synergistic effects with the nanopillars, different nanoparticles become more active for different wavelengths and locations on the pillars, providing broad-band enhancement. Nanostructured plasmonic silicon is a promising platform for integration with graphene and other 2D materials, for next-generation applications of large-area hybrid nanomaterials in the fields of sensing, photonics, optoelectronics, and medical diagnostics

Epitaxial 2D MoSe₂ (HfSe₂) Semiconductor/2D TaSe₂ Metal van der Waals Heterostructures

Molecular beam epitaxy of 2D metal TaSe₂/2D MoSe₂ (HfSe₂) semiconductor heterostructures on epi-AlN(0001)/Si(111) substrates is reported. Electron diffraction reveals an in-plane orientation indicative of van der Waals epitaxy, whereas electronic band imaging supported by first-principles calculations and X-ray photoelectron spectroscopy indicate the presence of a dominant trigonal prismatic 2H-TaSe₂ phase and a minor contribution from octahedrally coordinated TaSe₂, which is present in TaSe₂/AlN and TaSe₂/HfSe₂/AlN but notably absent in the TaSe₂/MoSe₂/AlN, indicating superior structural quality of TaSe₂ grown on MoSe₂. Apart from its structural and chemical compatibility with the selenide semiconductors, TaSe₂ has a workfunction of 5.5 eV as measured by ultraviolet photoelectron spectroscopy, which matches very well with the semiconductor workfunctions, implying that epi-TaSe₂ can be used for low-resistivity contacts to MoSe₂ and HfSe₂

Search for new particles in events with energetic jets and large missing transverse momentum in proton-proton collisions at root s=13 TeV

A search is presented for new particles produced at the LHC in proton-proton collisions at root s = 13 TeV, using events with energetic jets and large missing transverse momentum. The analysis is based on a data sample corresponding to an integrated luminosity of 101 fb(-1), collected in 2017-2018 with the CMS detector. Machine learning techniques are used to define separate categories for events with narrow jets from initial-state radiation and events with large-radius jets consistent with a hadronic decay of a W or Z boson. A statistical combination is made with an earlier search based on a data sample of 36 fb(-1), collected in 2016. No significant excess of events is observed with respect to the standard model background expectation determined from control samples in data. The results are interpreted in terms of limits on the branching fraction of an invisible decay of the Higgs boson, as well as constraints on simplified models of dark matter, on first-generation scalar leptoquarks decaying to quarks and neutrinos, and on models with large extra dimensions. Several of the new limits, specifically for spin-1 dark matter mediators, pseudoscalar mediators, colored mediators, and leptoquarks, are the most restrictive to date.Peer reviewe

Combined searches for the production of supersymmetric top quark partners in proton-proton collisions at root s=13 TeV

A combination of searches for top squark pair production using proton-proton collision data at a center-of-mass energy of 13 TeV at the CERN LHC, corresponding to an integrated luminosity of 137 fb(-1) collected by the CMS experiment, is presented. Signatures with at least 2 jets and large missing transverse momentum are categorized into events with 0, 1, or 2 leptons. New results for regions of parameter space where the kinematical properties of top squark pair production and top quark pair production are very similar are presented. Depending on themodel, the combined result excludes a top squarkmass up to 1325 GeV for amassless neutralino, and a neutralinomass up to 700 GeV for a top squarkmass of 1150 GeV. Top squarks with masses from 145 to 295 GeV, for neutralino masses from 0 to 100 GeV, with a mass difference between the top squark and the neutralino in a window of 30 GeV around the mass of the top quark, are excluded for the first time with CMS data. The results of theses searches are also interpreted in an alternative signal model of dark matter production via a spin-0 mediator in association with a top quark pair. Upper limits are set on the cross section for mediator particle masses of up to 420 GeV

Probing effective field theory operators in the associated production of top quarks with a Z boson in multilepton final states at root s=13 TeV

Peer reviewe

Measurements of the Electroweak Diboson Production Cross Sections in Proton-Proton Collisions at root s=5.02 TeV Using Leptonic Decays

The first measurements of diboson production cross sections in proton-proton interactions at a center-of-mass energy of 5.02 TeV are reported. They are based on data collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 302 pb(-1). Events with two, three, or four charged light leptons (electrons or muons) in the final state are analyzed. The WW, WZ, and ZZ total cross sections are measured as sigma(WW) = 37:0(-5.2)(+5.5) (stat)(-2.6)(+2.7) (syst) pb, sigma(WZ) = 6.4(-2.1)(+2.5) (stat)(-0.3)(+0.5)(syst) pb, and sigma(ZZ) = 5.3(-2.1)(+2.5)(stat)(-0.4)(+0.5) (syst) pb. All measurements are in good agreement with theoretical calculations at combined next-to-next-to-leading order quantum chromodynamics and next-to-leading order electroweak accuracy

Observation of tW production in the single-lepton channel in pp collisions at root s=13 TeV

A measurement of the cross section of the associated production of a single top quark and a W boson in final states with a muon or electron and jets in proton-proton collisions at root s = 13 TeV is presented. The data correspond to an integrated luminosity of 36 fb(-1) collected with the CMS detector at the CERN LHC in 2016. A boosted decision tree is used to separate the tW signal from the dominant t (t) over bar background, whilst the subleading W+jets and multijet backgrounds are constrained using data-based estimates. This result is the first observation of the tW process in final states containing a muon or electron and jets, with a significance exceeding 5 standard deviations. The cross section is determined to be 89 +/- 4 (stat) +/- 12 (syst) pb, consistent with the standard model.Peer reviewe

Measurement of the top quark mass using events with a single reconstructed top quark in pp collisions at root s=13 TeV

Abstract:A measurement of the top quark mass is performed using a data sample en-riched with single top quark events produced in thetchannel. The study is based on proton-proton collision data, corresponding to an integrated luminosity of 35.9 fb−1, recorded at√s= 13TeV by the CMS experiment at the LHC in 2016. Candidate events are selectedby requiring an isolated high-momentum lepton (muon or electron) and exactly two jets,of which one is identified as originating from a bottom quark. Multivariate discriminantsare designed to separate the signal from the background. Optimized thresholds are placedon the discriminant outputs to obtain an event sample with high signal purity. The topquark mass is found to be172.13+0.76−0.77GeV, where the uncertainty includes both the sta-tistical and systematic components, reaching sub-GeV precision for the first time in thisevent topology. The masses of the top quark and antiquark are also determined separatelyusing the lepton charge in the final state, from which the mass ratio and difference aredetermined to be0.9952+0.0079−0.0104and0.83+1.79−1.35GeV, respectively. The results are consistentwithCPTinvariance

Search for a heavy Higgs boson decaying into two lighter Higgs bosons in the tau tau bb final state at 13 TeV

A search for a heavy Higgs boson H decaying into the observed Higgs boson h with a mass of 125 GeV and another Higgs boson h(S) is presented. The h and h(S) bosons are required to decay into a pair of tau leptons and a pair of b quarks, respectively. The search uses a sample of proton-proton collisions collected with the CMS detector at a center-of-mass energy of 13TeV, corresponding to an integrated luminosity of 137 fb(-1). Mass ranges of 240-3000 GeV for m(H) and 60-2800 GeV for m(hS) are explored in the search. No signal has been observed. Model independent 95% confidence level upper limits on the product of the production cross section and the branching fractions of the signal process are set with a sensitivity ranging from 125 fb (for m(H) = 240 GeV) to 2.7 fb (for m(H) = 1000 GeV). These limits are compared to maximally allowed products of the production cross section and the branching fractions of the signal process in the next-to-minimal supersymmetric extension of the standard model.Peer reviewe