Structural and electronic properties of PTCDA single molecules and molecular layers on metal and semiconductor surfaces

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 22-10-200

Self-assembly of ClAlPc molecules on moiré-patterned graphene grown on Pt(111)

Phthalocyanines are promising molecules for the development of organic electronic devices, for instance, molecular heterojunctions in organic solar cells or organic field-effect transistors. For an optimum performance of these devices, the molecular ordering on the substrate and the molecular electronic level alignment have been shown as crucial factors. In this work, the self-assembled structure and the electronic structure of chloroaluminum phthalocyanines (ClAlPc) on graphene grown on Pt(111) surfaces have been studied by scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) and low-temperature conditions. Graphene grown on Pt(111) exhibits multiple moiré patterns with different periodicities, offering a benchmark to investigate the influence of the graphene and the moiré patterns in the ClAlPc ordering. This surface allows to extend previous works performed on graphite and graphene on Cu(100), where no moiré patterns are found. Well-ordered molecular islands exhibiting rotational domains have been observed in the submonolayer regime. The orientation of individual ClAlPc molecules within the structure unit cell has been characterized pointing out to a Cl-Up configuration adopted by the molecules. Our measurements show a correlation between the molecular lattice orientation and the graphene directions, whereas no influence of the underlying moiré patterns has been found. Finally, the ClAlPc electronic structure has been characterized indicating a weak graphene-molecule interactionFinancial support from the Spanish Ministerio de Economía y Competitividad (MINECO) and Fondo Europeo de Desarrollo Regional (FEDER) under grant No. MAT2016-77852-C2-2-R and from the Spanish Ministerio de Ciencia e Innovación, through the “María de Maeztu” Programme for Units of Excellence in R&D (grant No. CEX2018-000805- M) is gratefully acknowledge

Atomic-Scale Variations of the Mechanical Response of 2D Materials Detected by Noncontact Atomic Force Microscopy

We show that noncontact atomic force microscopy (AFM) is sensitive to the local stiffness in the atomicscale limit on weakly coupled 2D materials, as graphene on metals. Our large amplitude AFM topography and dissipation images under ultrahigh vacuum and low temperature resolve the atomic and moiré patterns in graphene on Pt (111), despite its extremely low geometric corrugation. The imaging mechanisms are identified with a multiscale model based on density-functional theory calculations, where the energy cost of global and local deformations of graphene competes with short-range chemical and long-range van der Waals interactions. Atomic contrast is related with short-range tip-sample interactions, while the dissipation can be understood in terms of global deformations in the weakly coupled graphene layer. Remarkably, the observed moiré modulation is linked with the subtle variations of the local interplanar graphene-substrate interaction, opening a new route to explore the local mechanical properties of 2D materials at the atomic scaleWe thank the Marie Curie ITN Network “ACRITAS” (Grant No. FP7-PEOPLE-2012-ITN-317348) funded by the European Commission under the FP7 Marie Curie PEOPLE programme and the Spanish MINECO (Projects No. CSD2010-00024, No. MAT2011-23627, No. MAT2013-41636-P, and No. MAT2014-54484-P) for financial support. Computer time was provided by the Spanish Supercomputer Network (RES) at Marenostrum III (BSC, Barcelona) and Magerit (CesViMa, Madrid) computers. P. P. was supported by the Ramón y Cajal progra

Built-up AFM tips by metal nanoclusters engineering

The ability to probe tip-sample interactions by Atomic Force Microscopy (AFM) has recently boosted our understanding of the matter at the atomic scale, enabling the study of properties of surfaces and adsorbates which were previously inaccessible. Nevertheless, this sensitivity to forces presents some drawbacks, as the requirement of a sharp tip apex to prevent the loss of spatial resolution due to the existence of long-range interactions. In this work, we have overcome this long-standing challenge by investigating the controlled extraction of single metallic nanoclusters, selectively grown on graphene. Our results show that the successive extraction of cluster allows to grow nanotips, which minimize the long-range tip-sample interactions and greatly enhance the topographic resolution. We have demonstrated that the created nanotips are very stable, which enables exchanging the sample and using the same nanotip to explore different surfaces without loss of resolution. Since metallic clusters of very different materials and sizes can be grown and selectively extracted by AFM, ours work paves also the way to the specific functionalization of AFM-tips to sense a large variety of interactionsFinancial support from the Spanish Ministerio de Economía y Competitividad (MINECO) and Fondo Europeo de Desarrollo Regional (FEDER) under grants No. MAT2016-77852-C2-2-R and MAT2016-80907-P and by the Comunidad de Madrid NMAT2D-CM program under grant S2018/NMT-4511 is gratefully acknowledged. Financial support from the Spanish Ministerio de Ciencia e Innovacion under grant Nº PID2019-106268GB-C31 is also gratefully acknowledged. We thank Rubén Pérez and Oscar Custance for helpful discussions and Antonio J. Martínez-Galera for helpful discussions and technical assistanc

Optimizing PMMA solutions to suppress contamination in the transfer of CVD graphene for batch production

Mass production and commercial adoption of graphene-based devices are held back by a few crucial technical challenges related to quality control. In the case of graphene produced by chemical vapor deposition, the transfer process represents a delicate step that can compromise device performance and reliability, thus hindering industrial production. In this context, the impact of poly(methyl methacrylate) (PMMA), the most common support material for transferring graphene from the Cu substrate to any target surface, can be decisive in obtaining reproducible sample batches. Although effective in mechanically supporting graphene during the transfer, PMMA solutions needs to be efficiently designed, deposited, and post-treated to serve their purpose while minimizing potential contaminations. Here, we prepared and tested PMMA solutions with different average molecular weight (AMW) and weight concentration in anisole, to be deposited by spin coating. Optical microscopy and Raman spectroscopy showed that the amount of PMMA residues on transferred graphene is proportional to the AMW and concentration in the solvent. At the same time, the mechanical strength of the PMMA layer is proportional to the AMW. These tests served to design an optimized PMMA solution made of a mixture of 550,000 (550k) and 15,000 (15k) AMW PMMA in anisole at 3% concentration. In this design, PMMA550k provided suitable mechanical strength against breakage during the transfer cycles, while PMMA-15k promoted depolymerization, which allowed for a complete removal of PMMA residues without the need for any post-treatment. An XPS analysis confirmed the cleanness of the optimized process. We validated the impact of the optimized PMMA solution on the mass fabrication of arrays of electrolyte-gated graphene field-effect transistors operating as biosensors. On average, the transistor channel resistance decreased from 1860 to 690 Ω when using the optimized PMMA. Even more importantly, the vast majority of these resistance values are distributed within a narrow range (only ca. 300 Ω wide), in evident contrast with the scattered values obtained in non-optimized devices (about 30% of which showed values above 1 MΩ). These results prove that the optimized PMMA solution unlock the production of reproducible electronic devices at the batch scale, which is the key to industrial productionproject "GEMIS – Graphene-enhanced Electro-Magnetic Interference Shielding", with the reference POCI-01-0247-FEDER-045939, co-funded by COMPETE 2020 – Operational Programme for Competitiveness and Internationalization and the Portuguese Foundation for Science and Technology (FCT), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF) and the FCT via the Strategic Funding UIDB/04650/2020. C. D. Liao acknowledges a Marie Skłodowska-Curie COFUND Fellowship (H2020-MSCA-COFUND 2015). T. Queirós acknowledges a PhD grant from FCT with reference SFRH/BD/150646/2020 in the framework of the Quantum Portugal Initiative. T. Domingues acknowledges a PhD grant from FCT with reference SFRH/BD/08181/202

Efficient reSe2 photodetectors with CVD single-crystal graphene contacts

Rhenium-based 2D transition metal dichalcogenides such as ReSe2 are suitable candidates as photoactive materials for optoelectronic devices. Here, photodetectors based on mechanically exfoliated ReSe2 crystals were fabricated using chemical vapor deposited (CVD) graphene single-crystal (GSC) as lateral contacts. A "pick & place" method was adopted to transfer the desired crystals to the intended position, easing the device fabrication while reducing potential contaminations. A similar device with Au was fabricated to compare contacts' performance. Lastly, a CVD hexagonal boron nitride (hBN) substrate passivation layer was designed and introduced in the device architecture. Raman spectroscopy was carried out to evaluate the device materials' structural and electronic properties. Kelvin probe force measurements were done to calculate the materials' work function, measuring a minimal Schottky barrier height for the GSC/ReSe2 contact (0.06 eV). Regarding the electrical performance, I-V curves showed sizable currents in the GSC/ReSe2 devices in the dark and under illumination. The devices presented high photocurrent and responsivity, along with an external quantum efficiency greatly exceeding 100%, confirming the non-blocking nature of the GSC contacts at high bias voltage (above 2 V). When introducing the hBN passivation layer, the device under white light reached a photo-to-dark current ratio up to 106.This research was funded by National Funds through the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2020 and projects PTDC/FIS-NAN/3668/2014 (LA2D) and PTDC/FIS-MAC/28114/2017 (POCI-01-0145-FEDER-028114) (GRAPHSENS). A.C. acknowledges the financial support of the project "GEMISGraphene-enhanced Electro-Magnetic Interference Shielding," with the reference POCI-01-0247-FEDER-045939, co-funded by COMPETE 2020-Operational Programme for Competitiveness and Internationalization and FCT-Science and Technology Foundation, under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF)

Cd and Cu interdiffusion in Cu(In,Ga)Se2/CdS hetero-interfaces

We report a detailed characterization of an industry-like prepared Cu(In,Ga)Se2 (CIGS)/CdS heterojunction by scanning transmission electron microscopy (STEM) and photoluminescence (PL). Energy dispersive x-ray spectroscopy (EDS) shows the presence of several regions in the CIGS layer that are Cu deprived and Cd enriched, suggesting the segregation of Cd-Se. Concurrently, the CdS layer shows Cd-deprived regions with the presence of Cu, suggesting a segregation of Cu-S. The two types of segregations are always found together, which, to the best of our knowledge, is observed for the first time. The results indicate that there is a diffusion process that replaces Cu with Cd in the CIGS layer and Cd with Cu in the CdS layer. Using a combinatorial approach we identified that this effect is independent of focused-ion beam sample preparation and of the TEM-grid. Furthermore, photoluminescence measurements before and after an HCl etch indicate a lower degree of defects in the post-etch sample, compatible with the segregates removal. We hypothesize that Cu2-xSe nanodomains react during the chemical bath process to form these segregates since the chemical reaction that dominates this process is thermodynamically favourable. These results provide important additional information about the formation of the CIGS/CdS interface.publishe

Role of sublimation kinetics of ammonia borane in chemical vapor deposition of uniform, large-area hexagonal boron nitride

Hexagonal boron nitride (h-BN) is a critical 2D insulator used as a substrate, gate dielectric, or encapsulation layer for graphene and other 2D materials and their van der Waals heterostructures. It is also promising as an active layer in single-photon emitters and other photonic devices. With the chemical formula H3N-BH3, ammonia borane is the most attractive precursor for up-scalable growth of large-area h-BN, using chemical vapor deposition given its stoichiometric B:N ratio, high stability under ambient conditions, nontoxicity, and high solubility in common solvents. Here, the synthesis of large-area (100 × 150 mm2) crystalline hexagonal boron nitride layers by thermal activation and decomposition of the precursor ammonia borane is presented. We describe two different reaction pathways for h-BN synthesis, providing evidence for dissimilarities in the sublimation kinetics of ammonia borane and how these differences critically influence the growth of h-BN. This understanding helps us accelerate h-BN production, reuse precursors, and reduce machine runtime, paving the way for upscalability. Moreover, our work provides a consistent unified view explaining the diverse deposition conditions reported in the literature for h-BN grown by CVD using ammonia borane as a precursor.This work was supported by National Funds through the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding No. UIDB/04650/2020 and Project Nos. PTDC/FIS-NAN/3668/2014 (LA2D) and PTDC/FIS-MAC/28114/2017 (POCI-01-0145-FEDER-028114) (GRAPHSENS)

Artifacts in time-resolved Kelvin probe force microscopy

Kelvin probe force microscopy (KPFM) has been used for the characterization of metals, insulators, and semiconducting materials on the nanometer scale. Especially in semiconductors, the charge dynamics are of high interest. Recently, several techniques for time-resolved measurements with time resolution down to picoseconds have been developed, many times using a modulated excitation signal, e.g., light modulation or bias modulation that induces changes in the charge carrier distribution. For fast modulation frequencies, the KPFM controller measures an average surface potential, which contains information about the involved charge carrier dynamics. Here, we show that such measurements are prone to artifacts due to frequency mixing, by performing numerical dynamics simulations of the cantilever oscillation in KPFM subjected to a bias-modulated signal. For square bias pulses, the resulting time-dependent electrostatic forces are very complex and result in intricate mixing of frequencies that may, in some cases, have a component at the detection frequency, leading to falsified KPFM measurements. Additionally, we performed fast Fourier transform (FFT) analyses that match the results of the numerical dynamics simulations. Small differences are observed that can be attributed to transients and higher-order Fourier components, as a consequence of the intricate nature of the cantilever driving forces. These results are corroborated by experimental measurements on a model system. In the experimental case, additional artifacts are observed due to constructive or destructive interference of the bias modulation with the cantilever oscillation. Also, in the case of light modulation, we demonstrate artifacts due to unwanted illumination of the photodetector of the beam deflection detection system. Finally, guidelines for avoiding such artifacts are given