Nano-Imaging of 2D Materials

Schematic illustration of charge carriers confined within a TMD flake comprising different thicknesses. Charge carriers in the ground state (blue) can be excited upon resonant light excitation to a higher state. (Source: F. Vialla, ICFO)

Semi­conducting hetero­structures have been key to the development of electronics and opto­electronics. Many appli­cations in the infrared and terahertz frequency range exploit transi­tions – inter­subband transi­tions – between quantized states in semi­conductor quantum wells. These intra­band transi­tions exhibit very large oscil­lator strengths, close to unity. Their discovery in III-V semicon­ductor hetero­structures depicted a huge impact within the condensed matter physics community and triggered the develop­ment of quantum well infrared photo­detectors as well as quantum cascade lasers.

Quantum wells of the highest quality are typically fabri­cated by molecular beam epitaxy. However, it poses two major limi­tations: lattice-matching is required, restric­ting the freedom in materials to choose from, and the thermal growth causes atomic diffu­sion and increases inter­face roughness. 2D materials can overcome these limi­tations since they naturally form a quantum well with atomi­cally sharp inter­faces. They provide defect free and atomi­cally sharp interfaces, enabling the forma­tion of ideal quantum wells, free of diffusive inhomo­geneities. They do not require epitaxial growth on a matching substrate and can therefore be easily isolated and coupled to other electronic systems such as Si CMOS or optical systems such as cavities and wave­guides.

Surpri­singly enough, inter­subband transi­tions in few-layer 2D materials had never been studied before, neither experi­mentally nor theoretically. Now, an inter­national research group around Frank Koppens from Insti­tute of Photonic Sciences ICFO in Barcelona reports on the first theo­retical calcu­lations and first experi­mental observation of inter-sub-band tran­sitions in quantum wells of few-layer semi­conducting 2D materials (TMDs). In their experiment, the team of researchers applied scattering scanning near-field optical micro­scopy (s-SNOM) as an inno­vative approach for spectral absorption measure­ments with a spatial resolution below 20 nm.

They exfo­liated TMDs, which comprised terraces of different layer thick­nesses over lateral sizes of about a few micro­meters. They directly observed the inter­subband reso­nances for these different quantum well thick­nesses within a single device. They also electro­statically tuned the charge carrier density and demonstrated intersubband absorp­tion in both the valence and conduction band. These obser­vations were comple­mented and supported with detailed theo­retical calcu­lations revealing many-body and non-local effects. The results pave the way towards an unex­plored field in this new class of materials and offer a first glimpse of the physics and tech­nology enabled by inter­subband transi­tions in 2D materials, such as infrared detectors, sources, and lasers with the potential for compact inte­gration with Si CMOS. (Source: ICFO)

Reference: P. Schmidt et al.: Nano-imaging of intersubband transitions in van der Waals quantum wells, Nat. Nano., online 27 August 2018; DOI: 10.1038/s41565-018-0233-9

Link: Quantum Nano-Optoelectronics (F. Koppens), Barcelona Institute of Science and Technology ICFO, Castelldefels, Spain

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