Microwave-Driven Exfoliation of Bulk 2H-MoS₂ after Acetonitrile Prewetting Produces Large-Area Ultrathin Flakes with Exceptionally High Yield

2D materials display exciting properties in numerous fields, but the development of applications is hindered by the low yields, high processing times, and impaired quality of current exfoliation methods. In this work we have used the excellent MW absorption properties of MoS2 to induce a fast heating that produces the near-instantaneous evaporation of an adsorbed, low boiling point solvent. The sudden evaporation creates an internal pressure that separates the MoS2 layers with high efficiency, and these are kept separated by the action of the dispersion solvent. Our fast method (90 s) gives high yields (47% at 0.2 mg/mL, 35% at 1 mg/mL) of highly exfoliated material (90% under 4 layers), large area (up to several μm2), and excellent quality (no significant MoO3 detected)

Fabrication of devices featuring covalently linked MoS2–graphene heterostructures

The most widespread method for the synthesis of 2D–2D heterostructures is the direct growth of one material on top of the other. Alternatively, flakes of different materials can be manually stacked on top of each other. Both methods typically involve stacking 2D layers through van der Waals forces—such that these materials are often referred to as van der Waals heterostructures—and are stacked one crystal or one device at a time. Here we describe the covalent grafting of 2H-MoS2 flakes onto graphene monolayers embedded in field-effect transistors. A bifunctional molecule featuring a maleimide and a diazonium functional group was used, known to connect to sulfide- and carbon-based materials, respectively. MoS2 flakes were exfoliated, functionalized by reaction with the maleimide moieties and then anchored to graphene by the diazonium groups. This approach enabled the simultaneous functionalization of several devices. The electronic properties of the resulting heterostructure are shown to be dominated by the MoS2–graphene interface.The authors acknowledge European Research Council (ERC-PoC- 842606 (E.M.P.); ERC-AdG-742684 (J. S.) and the MSCA program MSCA-IF-2019-892667 (N.M.S.), MINECO (CTQ2017-86060-P (E.M.P.) and CTQ2016-79419-R), Ministerio de Ciencia e Innovación (RTI2018-096075-A-C22 (E.B.), RYC2019-028429-I (E.B.)) the Comunidad de Madrid (MAD2D-CM S2013/ MIT-3007 (E.M.P.), Y2018/NMT-4783 (A.D.)) and the Programa de Atracción del Talento Investigador 2017-T1/IND-5562 (E.B.)). CzechNanoLab Research Infrastructure supported by MEYS CR (LM2018110) are gratefully acknowledged. IMDEA Nanociencia acknowledges support from the Severo Ochoa Programme for Centres of Excellence in R&D (MINECO, grant no. SEV-2016-0686).Peer reviewe

Double fingerprint characterization of uracil and 5-fluorouracil

Time Resolved Raman spectroelectrochemistry (TR-Raman-SEC) has been used for the first time to obtain two different Raman spectra of one single analyte in the same experiment. This double detection has been accomplished thanks to the use of electrochemical surface enhanced Raman scattering (EC-SERS) and electrochemical surface oxidation enhanced Raman scattering (EC-SOERS) in the same experiment. These two Raman enhancement phenomena can provide a broad insight into the interaction between analyte and substrate surface when they are combined. To prove the possibilities of this methodology, a Raman spectroelectrochemistry study of uracil (U) and 5-fluorouracil (5-FU), two analytes with relevance in medicine and biochemistry, have been performed. Density functional theory (DFT) calculations has been carried out to shed more light on the interaction of these molecules with silver substrates in acidic media.Ministerio de Economía, y Competitividad (Grant CTQ2017–83935-R-AEI/FEDERUE), Junta de Castilla y León (Grant BU297P18, Grant BU087G19, and Grant BU263P18) and Ministerio de Ciencia, Innovación y Universidades (Grant RED2018–102412-T and Grant PID2019–111215RB-I00). W. Ch. thanks JCyL for his postdoctoral fellowship (Grant BU297P18). S.H. thanks JCyL and European Social Fund for her predoctoral fellowship. M.P-E. thanks JCyL, the European Social Fund and the Youth Employment initiative and JCyL and European Social Fund for his predoctoral fellowship. This research has made use of the high-performance computing resources of the Castilla y León Supercomputing Center (SCAYLE, https://www.scayle.es), financed by FEDER (Fondo Europeo de Desarrollo Regional). Jorge Gonzalez is acknowledged for his help in the laboratory whose contract was founded by JCyL, the European Social Fund and the Youth Employment Initiative

Phase-Resolved Detection of Ultrabroadband THz Pulses inside a Scanning Tunneling Microscope Junction

Coupling phase-stable single-cycle terahertz (THz) pulses to scanning tunneling microscope (STM) junctions enables spatiotemporal imaging with femtosecond temporal and Angstrom spatial resolution. The time resolution achieved in such THz-gated STM is ultimately limited by the subcyde temporal variation of the tip-enhanced THz field acting as an ultrafast voltage pulse, and hence by the ability to feed high-frequency, broadband THz pulses into the junction. Here, we report on the coupling of ultra-broadband (1-30 THz) single-cycle THz pulses from a spintronic THz emitter (STE) into a metallic STM junction. We demonstrate broadband phase-resolved detection of the THz voltage transient directly in the STM junction via THz-field-induced modulation of ultrafast photocurrents. Comparison to the unperturbed far-field THz waveform reveals the antenna response of the STM tip. Despite tip-induced low-pass filtering, frequencies up to 15 THz can be detected in the tip-enhanced near-field, resulting in THz transients with a half-cycle period of 115 fs. We further demonstrate simple polarity control of the THz bias via the STE magnetization and show that up to 2 V THz bias at 1 MHz repetition rate can be achieved in the current setup. Finally, we find a nearly constant THz voltage and waveform over a wide range of tip-sample distances, which by comparison to numerical simulations confirms the quasi-static nature of the THz pulses. Our results demonstrate the suitability of spintronic THz emitters for ultrafast THz-STM with unprecedented bandwidth of the THz bias and provide insight into the femtosecond response of defined nanoscale junctions