Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping

Castellanos-Gómez, Andrés; Buscema, Michele; Molenaar, Rianda; Singh, Vibhor; Janssen, L.; Zant, Herre S. J. van der; Steele, Gary A.

doi:10.1088/2053-1583/1/1/011002

Cited by 1,735 publications

(1,673 citation statements)

References 27 publications

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“…18 We fabricate b-P based FET S by standard e-beam lithography, metal deposition (Ti/Au 5/50nm) and lift-off. In Figure 1a we show an optical micrograph of the fabricated b-P FET.…”

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confidence: 99%

Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors

et al. 2014

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Few-layer black phosphorus, a new elemental 2D material recently isolated by mechanical exfoliation, is a high-mobility layered semiconductor with a direct bandgap that is predicted to strongly depend on the number of layers, from 0.35 eV (bulk) to 2.0 eV (single-layer). Therefore, black phosphorus is an appealing candidate for tunable photodetection from the visible to the infrared part of the spectrum. We study the photoresponse of field-effect transistors (FETs) made of few-layer black phosphorus (3 nm to 8 nm thick), as a function of excitation wavelength, power and frequency. In the dark state, the black phosphorus FETs can be tuned both in hole and electron doping regimes allowing for ambipolar operation. We measure mobilities in the order of 100 cm 2 /V s and current ON/OFF ratio larger than 10 3 . Upon illumination, the black phosphorus transistors show response to excitation wavelengths from the visible up to 940 nm and rise time of about 1 ms, demonstrating broadband and fast detection. The responsivity reaches 4.8 mA/W and it could be drastically enhanced by engineering a detector based on a PN junction. The

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“…18 We fabricate b-P based FET S by standard e-beam lithography, metal deposition (Ti/Au 5/50nm) and lift-off. In Figure 1a we show an optical micrograph of the fabricated b-P FET.…”

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confidence: 99%

Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors

et al. 2014

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“…Our samples were prepared by mechanical exfoliation of WSe 2 bulk crystals (commercially acquired from HQgraphene) on viscoelastic substrates and were subsequently transferred onto diamond. 40 Monolayer flakes with typical diameters of the order of 100 μm were identified by photoluminescence and reflectance contrast spectroscopy. An overview of the basic experimental concept is schematically illustrated in the inset of Figure 1b.…”

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Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe₂

et al. 2017

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Many of the fundamental optical and electronic properties of atomically thin transition metal dichalcogenides are dominated by strong Coulomb interactions between electrons and holes, forming tightly bound atom-like states called excitons. Here, we directly trace the ultrafast formation of excitons by monitoring the absolute densities of bound and unbound electron−hole pairs in single monolayers of WSe 2 on a diamond substrate following femtosecond nonresonant optical excitation. To this end, phaselocked mid-infrared probe pulses and field-sensitive electro-optic sampling are used to map out the full complex-valued optical conductivity of the nonequilibrium system and to discern the hallmark low-energy responses of bound and unbound pairs. While the spectral shape of the infrared response immediately after above-bandgap injection is dominated by free charge carriers, up to 60% of the electron−hole pairs are bound into excitons already on a subpicosecond time scale, evidencing extremely fast and efficient exciton formation. During the subsequent recombination phase, we still find a large density of free carriers in addition to excitons, indicating a nonequilibrium state of the photoexcited electron−hole system. KEYWORDS: Dichalcogenides, atomically thin 2D crystals, exciton formation, ultrafast dynamics A tomically thin transition metal dichalcogenides (TMDCs) have attracted tremendous attention due to their direct bandgaps in the visible spectral range, 1,2 strong interband optical absorption, 3,4 intriguing spin-valley physics, 5−7 and applications as optoelectronic devices. 8−11 The physics of twodimensional (2D) TMDCs are governed by strong Coulomb interactions owing to the strict quantum confinement in the out-of-plane direction and the weak dielectric screening of the environment. 12,13 Electrons and holes in these materials can form excitons with unusually large binding energies of many hundreds of millielectronvolts, 14−19 making these quasiparticles stable even at elevated temperatures and high carrier densities. 20,21 The properties of excitons in 2D TMDCs are a topic of intense research, investigating, for example, rapid exciton−exciton scattering, 22 interlayer excitons, 23 charged excitons and excitonic molecules, 24,25 ultrafast recombination dynamics, 19,26−28 or efficient coupling to light and lattice vibrations. 4,19,29,30 In many experiments, excitons are created indirectly through nonresonant optical excitation or electronic injection, which may prepare unbound charge carriers with energies far above the exciton resonance. 8,18 Subsequently, the electrons and holes are expected to relax toward their respective band minima and form excitons in the vicinity of the fundamental energy gap. In principle, strong Coulomb attraction in 2D TMDCs should foster rapid exciton formation. Recent optical pump−probe studies relying on interband transitions have reported characteristic formation times on subpicosecond time-scales. 31 The relaxation of large excess energies, however, requires many sca...

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“…For this, we use Gel-Film T M pieces (Gelpak) as an intermediate substrate, and suitable monolayer flakes are then stamped onto the final substrate (silicon covered with a 285 nm SiO 2 layer and pre-defined metal markers) using an all-dry transfer technique 24 . The investigated samples are identified as HQ (monolayer flake prepared from HQ graphene MoSe 2 bulk crystal) and n-2D (monolayer flake prepared from n-doped 2D semiconductors MoSe 2 bulk crystal) throughout the manuscript.…”

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Long-lived spin polarization in n-doped MoSe2 monolayers

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Hanninger

et al. 2017

Applied Physics Letters

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Transition metal dichalcogenide monolayers are highly interesting for potential valleytronic applications due to the coupling of spin and valley degrees of freedom and valley-selective excitonic transitions. However, ultrafast recombination of excitons in these materials poses a natural limit for applications, so that a transfer of polarization to resident carriers is highly advantageous. Here, we study the low-temperature spin-valley dynamics in nominally undoped and n-doped MoSe 2 monolayers using time-resolved Kerr rotation. In the ndoped MoSe 2 , we find a long-lived component of the Kerr signal which we attribute to the spin polarization of resident carriers. This component is absent in the nominally undoped MoSe 2 . The long-lived spin polarization is stable under applied in-plane magnetic fields. Spatially resolved measurements allow us to determine an upper boundary for the electron spin diffusion constant in MoSe 2 .Transition metal dichalcogenide (TMDC) monolayers have a peculiar band structure 1,2 , in which spin and valley degrees of freedom are coupled, and the optical selection rules allow for valley-selective generation of excitons 3 . An excitonic valley polarization can be read out optically via the circular polarization degree of the emitted photoluminescence (PL), and initial studies using continuous-wave excitation revealed a large steadystate polarization 4,5 , which was shown to be stable against depolarization in large in-plane magnetic fields 6 . While many early studies of valley physics focused on the naturally abundant MoS 2 , synthetic TMDC crystals quickly garnered scientific attention due to their spectrally narrow PL emission 7 , and the large tuning range of conduction-and valence-band spin splitting 8-10 afforded by the different combinations (or alloys 11,12 ) of the constituent elements Mo, W, S, Se, and Te. Time-resolved studies of exciton recombination and valley dynamics showed that ultrafast radiative recombination 13-17 is partially responsible for the large valley polarization observed in experiments, as the exciton lifetime limits the time window for valley dephasing. However, the ultrafast excitonic recombination also severely limits potential applications of the valley degree of freedom in novel devices, so that a transfer of valley polarization to resident carriers, which was observed in several TMDCs 18,19 , is highly advantageous. Among the TMDC family, MoSe 2 is characterized by a comparatively low optically induced valley polarization degree, which is only observable at all under near-resonant excitation conditions 20-22 and can be increased by modifying the recombination dynamics using coupling to photonic cavities 23 . To investigate the anomalously low valley polarization in MoSe 2 , timeresolved studies of valley dynamics are needed.Here, we directly compare the low-temperature spinvalley dynamics in nominally undoped and n-doped MoSe 2 monolayers using time-resolved Kerr rotation (TRKR). We find a long-lived component of the Kerr a) Electronic mail: tobias.ko...

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Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping

Cited by 1,735 publications

References 27 publications

Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors

Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors

Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe₂

Long-lived spin polarization in n-doped MoSe2 monolayers

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Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping

Cited by 1,735 publications

References 27 publications

Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors

Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors

Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe2

Long-lived spin polarization in n-doped MoSe2 monolayers

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Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe₂