Light-emitting diodes by band-structure engineering in van der Waals heterostructures

Abstract

2The class of 2D atomic crystals 1 , which started with graphene 2 now includes a large variety of materials. However, the real diversity can be achieved if one starts to combine several such crystals in van der Waals heterostructures 3,8 . Most attractive and powerful is the idea of band-structure engineering, where by combining several different 2D crystals one can create a designer potential landscape for electrons to live in. Rendering the band-structure with atomic precision allows tunnel barriers, QWs and other devices, based on the broad choice of 2D materials.Such band-structure engineering has previously been exploited to create LEDs and lasers based on semiconductor heterostructures grown by molecular beam epitaxy 9 . Here we demonstrate that using graphene as a transparent conductive layer, hBN as tunnel barriers and different transition metal dichalcogenides (TMDC) 1,10 as the materials for QWs, we can create efficient LEDs; Fig. 1F. In our devices, electrons and holes are injected to a layer of TMDC from the two graphene electrodes.Because of the long lifetime of the quasiparticles in the QWs (determined by the height and thickness of the neighbouring hBN barriers), electrons and holes recombine, emitting a photon. The emission wavelength can be fine-tuned by the appropriate selection of TMDC and quantum efficiency (QE) can be enhanced by using multiple QWs (MQWs).We chose TMDC because of wide choice of such materials and the fact that monolayers of many TMDC are direct band gap semiconductors [11][12][13][14][15] . Until now, electroluminescence (EL) in TMDC devices has been reported only for lateral monolayer devices and attributed to thermally assisted processes arising from impact ionization across a Schottky barrier 16 and formation of p-n junctions 15,17,18 /hBN. (H-J) Band diagrams for the case of zero applied bias (H), intermediate applied bias (I) and high bias (J) for heterostructure presented in (G). 4For brevity we concentrate on current-voltage (I-V) characteristics, photoluminescence (PL) and EL spectra from symmetric devices based on MoS 2 , Fig At low V b , the PL in Fig. 2A is dominated by the neutral A exciton, X 0 , peak 12 at 1.93 eV. We attribute the two weaker and broader peaks at 1.87 and 1.79 eV to bound excitons 22,23 . At certain V b , the PL spectrum changes abruptly with another peak emerging at 1.90 eV. This transition is correlated with an increase in the differential conductivity ( Fig. 2A). We explain this transition as being due to the fact that at this voltage the Fermi level in Gr B rises above the conduction band in MoS 2 , allowing injection of electrons into the QW (Fig. 1I). This allows us to determine the band alignment between In contrast to PL, EL starts only at V b above a certain threshold, Figs. 2B. We associate such behaviour with the Fermi level of Gr T being brought below the edge of the valence band so that holes can be injected to MoS 2 from Gr T (in addition to electrons already injected from Gr B ) as sketched in Fig. 1J. This creates con...

Self-Aligned van der Waals Heterojunction Diodes and Transistors

“…With previously reported fabrication methods, p-n vdWHs, whether lateral or vertical, consisted of a p-n heterojunction connected by two lateral p-type and n-type extensions (acting as FETs in series) or Schottky diodes with graphene, with the overall stack being coupled to one or two gates with alignment errors increasing with each component. [7][8][9][26][27][28][29][30][31] In the lateral geometry, p-n vdWHs offer electrostatically controlled doping in the constituent semiconductors but suffer from large parasitic resistance from the lateral extensions beyond the junction region. [7][8][9][28][29][30] On the other hand, vertical p-n vdWHs that employ a graphene electrode can achieve larger current density at the cost of defect-induced leakage currents, extraneous Schottky barriers, and electrode screening issues.…”

“…[7][8][9][28][29][30] On the other hand, vertical p-n vdWHs that employ a graphene electrode can achieve larger current density at the cost of defect-induced leakage currents, extraneous Schottky barriers, and electrode screening issues. 27,30,31 For example, fully vertical BP-MoS2 and WSe2-MoS2 p-n vdWHs using graphene contacts show poor electrostatic control of ID-VTG characteristics (Supporting Fig. S2.7).…”

Independent Band Modulation in 2D van der Waals Heterostructures via a Novel Device Architecture

“…Recently, considerable research interest has been intrigued by the vertically stacked vdWs integration of various 2DLMs, which provides infinite possibilities by overcoming the limitation of lattice matching and processing compatibility 6, 7, 8, 9, 10, 11, 12, 13, 14. Among various categories of vertically stacked vdWs heterostructured devices, the tunneling field effect transistor (TFET), which provides a promising sub‐60‐mV dec −1 subthreshold swing (SS), has been regarded as a promising application of vdWs heterostructure for future energy‐efficient electronics 15, 16, 17, 18, 19…”

Reversible Mechanochromic Delayed Fluorescence in 2D Metal–Organic Micro/Nanosheets: Switching Singlet–Triplet States through Transformation between Exciplex and Excimer

“…During last few years, the 2D layered structures have received much attention due to the unique properties and applications in the fields of opto/electronic transistors,13 LEDs,14 (electro)chemical sensors,15 solar cells,16 and plasmon–photonic polaritons 17. In theory, the weak interactions between 2D stacked monolayers could facilitate the slipping and deformation of molecular sheets under the external force, which favors the adjustment of arrangement and interactions between chromophores toward force‐responsive characteristics 18.…”