Spatially controlled two-dimensional quantum heterostructures

Figures & data

Figure 1. Quantum confinement in 2D materials. Schematics showing (a) the exciton Bohr radius [7] and (b) the diffusion length [12, 13, 14], and (c) the mean free path of free electrons [15, 21]. They are three factors to be considered for designing and engineering the quantum confinement effects in 2D materials.

Figure 2. Approaches for spatially controlled growth of 2D quantum heterostructures. (a) Schematic of ‘Sequential epitaxy’ process, in which a new 2D layer is formed either vertically or laterally onto the pre-deposited 2D material by sequentially supplying sources. (b) Schematic of ‘Etching and regrowth’ process, in which a part of a 2D material is etched and then filled with another material via a chemical reaction. (c) Schematic of ‘Conversion’ technique for converting a preformed crystal into another crystal by a source supply during the thermal CVD. (d) Schematic of ‘One-pot growth’ process, showing a growth method that simultaneous growth of 2D heterostructure with different precursors in the same reactor.

Table 1. Summary of the spatially controlled growth of 2D lateral and vertical heterostructures. Lateral and vertical heterostructures are denoted by A-B and A/B, respectively.

Growth method	Heterostructures	Growth direction	Minimum feature size	Resultant crystal structures	Device applications or quantum phenomena	Refs
Sequential epitaxy	MoS2-WSe2	Lateral /Vertical	∼10 μm	Controllable lateral/vertical growth aspects		[26]
			∼1 μm			[27]
	GaSe-MoS2		∼3 μm		Photodetectors	[28]
	MoSe2-WSe2		∼3 μm			[29]
	Graphene-hBN	Lateral	80 nm	Lateral heteroepitaxy		[30]
	WS2-WSe2, WSe2-MoS2, WSe2-MoSe2		∼200 nm	Lateral superlattices of quantum well		[31]
	WS2-WSe2		∼15 nm		Strain engineering of band structure	[32]
	MoSe2-WSe2		∼10 nm	Lateral heterostructure flake	Self-powered photodetector	[33]
	WSe2-MoS2		A few μm	Self-aligned heterojunction	Light-emitting diodes	[34]
			∼150 nm	Lateral heterostructure flake	Strain-induced type I band alignment	[35]
	PtTe2-MoS2		∼10 μm	Position-controlled heterostructure arrays	Transistors with low contact resistance and sub-1-nm transfer length	[36]
	MoS2/WS2	Vertical	Monolayer	Vertical superlattices	Valley-polarized interlayer excitation	[37]
	WSe2/Bi2Se3		3.2 nm	Vertically stacked heterostructure	Self-powered broadband photodetector	[38]
	SnS2/WSe2, NbSe2/MoSe2, SnS2/MoS2/WSe2		Monolayer	Roll-up superlattices	1D magneto-transport	[39]
	Graphene/MoS2		N/A	Lattice aligned MoS2 on graphene	Band gap narrowing of MoS2	[40]
Etching and regrowth	WS2-WSe2, WSe2-MoS2, WSe2-MoS2	Lateral	∼3 μm	Mosaic heterostructure		[41]
	Graphene-hBN		15 nm	Graphene nanoribbon	Carrier mobility of 20000 cm^2/Vs	[42]
	Graphene-hBN		6 nm	Graphene nanoribbon	Bandgap with on/off ratios > 10^4	[43]
	Graphene-hBN		5 nm	Controllable edge topologies	Band gap opening and magnetic conductance	[44]
	VSe2/WSe2, NiTe2/WSe2, CoTe2/WSe2, NbTe2/WSe2, VS2/WSe2, VSe2/MoS2, VSe2/WS2	Vertical	N/A	Arrays of vertical heterostructure	High ON-current density transistors	[45]
	Graphene/hBN					[46]
	Graphene/MoS2			Heterostructure with a vertically overlapped MSJ (2–200 nm)	Transistors with low contact resistance	[47]
						[48]
Post-synthetic conversion	MoS2-MoSe2	Lateral	∼3 μm	Arrays of lateral heterostructure	Type-I n-n junction	[49]
	MoS2-Mo2C		∼25 nm		Transistor with low contact resistance	[50]
	Graphene-hBN		∼1 μm		Graphene transistors with an on/off ratio of ∼10^3	[51]
			∼25 μm			[52]
			∼7 nm		Band splitting of 80–160 meV	[53]
					Blue emission at the interface	[54]
	MoS2-WSe2	Lateral	∼ 2 nm	Sub-nm-wide 1D MoS2 channels		[55]
Simultaneous one-pot synthesis	1T’-MoTe2-2H-MoTe2	Lateral	A few μm	Patternable lateral heterostructure	Transistor with low contact resistance	[56]
	MoS2-WS2		∼3 μm	Monolayer lateral heterostructure	Self-powered photodetector	[57]
	WS2/MoS2	Lateral/Vertical	∼5 μm	Controllable lateral/vertical growth aspects	p–n junction and photovoltaic effect	[58]
	Mo2C/Graphene	Vertical	Monolayer	Controlled growth of graphene depending on carbon flux	Hydrogen evolution reaction	[59]

Figure 3. Sequential epitaxial growth of vertical/lateral 2D heterostructures. (a, b) Determination of lateral/vertical growth aspects by the W/Se ratio during the two-step CVD [26]. (a) Schematic of controllable lateral edge-mediated growth and vertical vdW epitaxy of heterostructures composed of MoS₂ and WSe₂ depending on the adsorption of various active clusters of W₁Se_x. (b) The diffusion barriers of W₁Se₃ and W₁Se₁ clusters on the edge/basal surface of the MoS₂ layer. (c-e) Edge-mediated epitaxy of lateral WS₂-WSe₂-WS₂ heterostructure [31]. (c) Schematic of the edge-mediated epitaxy of lateral heterostructures. (d) High-resolution scanning transmission electron microscopy (STEM) image of the WS₂-WSe₂ lateral heterostructure (left). Magnified STEM image of the dashed red rectangle region (top right) and the corresponding atomic model (bottom right). (e) Scanning electron microscopy (SEM) images of the lateral WS₂-WSe₂-WS₂ multi-heterojunction with ultra-narrow WSe₂ width (∼35 and 13 nm) with different growth times (10 and 5 s). (f) vdW epitaxy of vertical MoS₂/WS₂ heterostructures [37]. Flow-rate modulations of the metal-organicprecursors for the growth of (top) and a series of cross-sectional high-angle annular dark field (HAADF)-STEM images of the heterostructures depending on the flow rate (bottom) are shown. (g-i) Capillary force-driven rolling-up strategy for the fabrication of vdW superlattice [39]. (g) Schematic illustration of the fabrication process of roll-up vdW superlattices. (h) STEM image of a rolled-up SnS₂/WSe₂ heterostructure. (i) STEM image (left) and corresponding energy-dispersive spectroscopy (EDS) mapping images (center: W, right: Sn) of the SnS₂/WSe₂ vdW superlattice.

Figure 4. Etching and regrowth process for the production of vertical/lateral 2D heterostructures. (a, b) Synthesis of periodic monolayer 2D heterostructures by laser-pattering and thermal etching process [41]. (a) Schematic illustration of the growth process consists of laser patterning and thermal etching of the pre-deposited WS₂ matrix followed by the growth of WSe₂. (b) Optical microscopy (OM) image of a lateral heterostructure with WSe₂ crystals embedded in a monolayer WS₂ matrix. (c, d) Synthesis of chirality-controlled graphene nanoribbons (GNR) embedded in hBN [44]. (c) Schematic showing the strategy to prepare GNRs with crystallographic edge orientations. Different particles of Ni or Pt etch nano-trenches with different edge topologies (Ni: zigzag, Pt: armchair) in an hBN layer. (d) AFM image of 1D GNR with zigzag terminations embedded in 2D hBN. (e, f) Vertical growth of VSe₂ on WSe₂ by incomplete etching of the WSe₂ [45]. (e) Schematic of the vertical growth process by laser irradiation. (f) OM image of periodic arrangements of VSe₂/WSe₂ heterostructure arrays.

Figure 5. Post-synthetic conversion process to achieve lateral 2D heterostructures. (a, b) Position-controlled conversion of 2D materials for the growth of lateral MoS₂-MoS₂ heterostructures [49]. (a) Patterning and selective conversion of 2D materials using SiO₂ masking layers for MoSe₂-MoS₂ heterostructures. (b) Raman mappings of MoS₂-MoSe₂ lateral heterojunction arrays formed within monolayer crystals by patterning and selective conversion processes. (c-e) Spatially controlled conversion process from hBN to graphene [52, 53]. (c) Schematic illustrations for the conversion process of hBN on Pt substrate to produce 2D lateral heterostructure of graphene-hBN [52]. (d, e) Schematic (d) and SEM image (e) of graphene quantum dots embedded in the hBN matrix, grown by the selective conversion on an array of the Pt nanoparticles. [53]. (f) Schematic of the patterning process guided by misfit dislocations (marked as ‘T’) at the MoS₂-WSe₂ lateral heterojunction. (g) STEM images of MoS₂ 1D channels embedded within WSe₂. (h) STEM image (left) and strain map (right) of a MoS₂ 1D channel formed from an intrinsic 5|7 dislocation in WSe₂ [55].

Figure 6. Simultaneous, one-pot synthesis of vertical or lateral 2D heterostructures. (a, b) Simultaneous integration of metallic 1T’-MoTe₂ and semiconducting 2H-MoTe₂ by tellurization of patterned precursors of MoO₃ and MoO_3-x [56]. (a) Schematic for the growth of lateral heterostructure of 1T’−2H MoTe₂ heterojunction. (b) STEM image of the heterojunction (inset: SEM image). (c, d) Direct synthesis of vertical 2D Mo₂C/graphene heterostructure using molten Mo-Cu alloy catalyst and high CH₄ concentration during the CVD [59]. (c) Schematic of the growth process, and (d) OM image of Mo₂C crystals on a graphene monolayer.

Figure 7. Promising applications of vertical or lateral 2D heterostructures. (a, b) Light-emitting diode (LED) based on vertical heterostructures composed of hBN, graphene, and TMD layers [70]. (a) Schematic of band alignments, and (b) OM image showing the red-light emission from the LED of vdW heterostructure. (c, d) Self-powered photodetector fabricated using p-n Bi₂Te₃-WSe₂ heterostructure [38]. (c) OM image of the photodetector, and (d) photoresponse of the corresponding junction device with different light wavelengths (375, 520, 980, and 1550 nm). (e, f) Tunneling transistor application realized by using graphene quantum dots (GQD) embedded in hBN layers [53]. (e) Modeling of the single-electron charging effect. (f) Conductance (G) as a function of V_g and V_b for a device with 10 nm-sized graphene quantum dots (QDs) measured at T = 0.25 K. (g, h) Quantum-confined electronic states in MoS₂/WSe₂ heterobilayers [82]. (g) STM image and (h) conductance map taken along the yellow line in (g) for voltages in the valence band-edge region. (i, j) Lateral metal-semiconductor junction (MSJ) comprised of MoS₂ and PtTe₂ synthesized by two-step growth [36]. (i) Schematic (top) and OM image (bottom) of the heterostructure. (j) Output electrical characteristics of MSJ transistor where the carriers are injected from PtTe₂ (red) and Ti (blue). (k) The proposed quantum-confined heterostructure based on 0D QDs and 2D matrix with type-I band alignment, (left) the atomic schematic, and (right) the band structure.

Figure 8. Lattice and interlayer spacing mismatch between TMDs. The values in yellow boxes on the central diagonal represent the lattice constant (top, blue) and interlayer spacing (bottom, red) of the TMDs, respectively. The values in the lower right (blue) and upper left boxes (red) are the lattice and spacing mismatch between each TMD, of which values less than 3% are highlighted. The lattice mismatch was calculated by simply considering the in-plane lattice distances of each unit cell of two materials.

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