Abstract
1. Introduction
From the successful fabrication of atomically thin carbon films by Andre Geim and Konstantin Novoselov through mechanical exfoliation in 2004[
Undoubtably, as the first discovered and the most popular 2D material, graphene has attracted the most attention on its physical properties and the exploration for more potential applications. For instance, recently, graphene was even used as a holder for transmission electron microscopy[
In 2007, Lopes dos Santos et al.[
In next three years, from the twist-angle disorder in TBG[
Analogously, in twisted bilayer transition metal dichalcogenides (TBTMDs) structures, owing to the comparability between Coulomb interactions and narrow bandwidth, flat bands[
Table 1 performs a simple summary for primary 2D materials discussed in this review with their typical band structure and some important as well as intriguing physical properties, providing a generalized prospect for the following description.
It is well known that artificially controlling the twist-angle between two stacked materials actually violates the lowest energy principle of the whole system, especially some weird angles like the magic angle. One-step CVD under special conditions could produce twisted layers but with uncontrollable as well as random twist-angles[
When increasing progress has been made in exploring electronic structure changing, interlayer coupling and excitonic behavior in the twisted 2D systems, relevant applications in microelectronics and optoelectronics, such as rectifying and response to incident radiation, have also obtained increasing concerns. In this article, we dominantly focus on twist-angle two-dimensional superlattices and their applications in (opto)electronics. Some representative works containing versatile applications (i.e., photodetector, diode, field-effect transistor, Josephson junction…) and diverse materials (i.e., graphene, TMDs, BP, h-BN) will be exhibited. Therefore, Moiré pattern, interlayer coupling and interlayer exciton behaviors in different 2D materials will be discussed primarily. Then, twist-angle-dependent Moiré bands and anisotropy of the twisted superlattice will give rise to tunable (opto)electrical properties, resulting in versatile applications including a transistor, rectifier, photodetector and light emission diode.
2. Moiré superlattice
Because of the convenient operation and superior controllability of twisting, which can maintain intrinsic features and bring novel properties simultaneously, twist-induced Moiré pattern in the superlattice became an outstanding platform for study on twist-angle-dependent properties[
When focusing on the detailed atomic arrangement in twisted 2D systems, it is notable that small domains inside a Moiré periodicity possess versatile stacking modes. Compared to uniform Bernal stacking (AB stacking) in bilayer graphene without twisting, TBG exhibits much more complex and diverse stacking modes including AA, AB, BA, SP…, which are typically shown as alternately triangular or hexagonal regimes with interval domain walls[
On one hand, owing to the different spatial configuration in different stacking modes, carriers moving in the Moiré superlattice will feel like a different chemical environment, resulting in the imparity of electronic structure and the presence of periodic Moiré potentials in real space which will exert great influence on carrier transportation (Figs. 4(a) and 4(b)). On the other hand, from the perspective of momentum space, the Moiré superlattice with a much larger scale is corresponded to smaller Brillouin zone due to the reciprocal relation between real space and momentum space[
Actually, the Moiré superlattice was seen as the ideal platform to investigate exciton behaviors a long time ago, typically using PL spectra to check the excitonic energy (Figs. 4(c) and 4(d)) and Raman spectrum to testify the interlayer coupling (Figs. 4(e)–4(h). In 2014, Liu et al.[
Intriguing phenomenon in the PL spectrum implied attractive exciton behaviors in the Moiré superlattice. In 2018, the arcane Moiré potentials and the behavior of Moiré-potential-trapped excitons were detected experimentally in twisted MoSe2/WSe2 heterostructures by Seyler et al.[
In short, twisting, as a new freedom degree of adjusting properties of the twist-angle 2D systems, not only tailors the electronic structure of charge carriers inside but also creates Moiré potential to change the exciton behavior. Therefore, for a twist-angle 2D Moiré superlattice, band structure, photoinduced transition routes, interlayer coupling strength and periodic potential field can all be easily tuned by twist-angle combining with external electric or magnetic field, indicating the twisted system is a significantly promising candidate for applications in (opto)electronic devices. Hence, some realized achievements and theoretical predictions will be discussed later in this article.
3. Graphene-based devices
Graphene, as the first discovered and the most popular 2D material, has owned abundant attention on its twistronic properties, hence going the farthest in applications among other twisted 2D counterparts.
Since the atomic thickness endows transparency to graphene, the low photoresponsivity partially caused by the low photon absorption ~2.3% and the inferior selectivity (from ultraviolet to infrared) become a bottleneck for optimizing monolayer graphene-based photodetectors. Therefore, in 2013, Bao et al.[
In 2016, considering the twist-angle-dependent band structure, Yin et al.[
After obtaining TBG with various twist angles by CVD growth on copper foil and subsequently transferred to the SiO2/Si substrate, a photodetector was fabricated with two regimes in which bilayer graphene were 7° and 13° twisted, respectively (Figs. 6(a) and 6(b)). Then a significant enhancement of Raman G-band intensity and photocurrent in 13° TBG than 7° was observed when being shined by incident light with a wavelength of 532 nm (2.33 eV), which can be attributed to the well matching of the gap value (2EVHS = 2.34 eV in TBG at 13°) with incident photon energy (Figs. 6(c) and 6(d)). It was evidenced by the fact that the photoresponsivity of 13° TBG under 532 nm, which reached 1 mA/W, was nearly seven-fold larger than that of 7° TBG. By changing the twist-angle, TBG exhibited a strong photocurrent to incident light with different certain wavelength, indicating the selectivity in photoresponsivity was achieved by twisting (Fig. 6(e)).
In the same year, this group improved synthesis method by combining CVD with PMMA transfer in order to obtain large-domain TBG with the size around 100 μm (Fig. 6(f)). For verifying the correlation between enhancement in the selectivity of photoresponsivity and the DOS gap in Moiré bands, Tan et al.[
Similar to Zhongfan Liu’s work, Xin et al.[
Photoresponsivity in the terahertz (THz) range with polarization-sensitivity to different twist-angles was theoretically predicted in twisted bilayer graphene quantum dots (GQD) in 2019 by Tiutiunnyk et al.[
Actually, the terahertz photo-galvanic response was realized earlier in the TBG photodetector instead of twisted GQDs. In 2020, Otteneder et al.[
The enhanced light-matter interaction in TBG unveiled by Yin et al.[
It was fully evident that aforementioned studies utilized twisting to promote the merging between two Dirac cones in order to obtain VHSs in DOS which played a critical role in optimizing the interaction of light and matter. In addition, twist-angle-dependent Moiré bandgap between Moiré Dirac bands and upper (or lower) bands will offer another way to introduce transition processes.
Different with relatively large twisting, Deng et al.[
For a photodetector made by 1.81° TBG, light-matter interaction could be significantly enhanced through bolometric effect under incident light with a wavelength of 12 μm than 5.0 and 7.7 μm. It was observed that the photoresponsivity reached 26 mA/W under 1200 nm radiation when the Fermi level was tuned in the center of the superlattice gap under gate bias of 43.5 V. Similarly, when the gate bias was adjusted to –43.5 V, indicating the Fermi level was tuned in the gap of hole branch and the Moiré band was vacated, photoresponsivity and sheet resistance also reached a maximum. Otherwise, responsivity and resistance would decrease fast, resulting from the metallic behavior when the Moiré band was half-filled since EF was positioned in the superlattice band. However, it was notable that such strong photoresponsivity finally disappeared with continuously decreasing twist-angle due to the close of superlattice bandgap.
When we pay attention from photoexcited transition to the tunable band structure in TBG itself, it reminds of the possibility in electronic applications. The requirements for electronic devices are totally different from optoelectronics because bandgap engineering will make a limited contribution to electric properties like current rectifying but tunable band structure provides an idea to break band symmetry to generate anisotropy in effective mass by exerting external fields.
In 2021, Liu et al.[
When magic-angle twisted bilayer graphene (MATBG) is paid attention from the investigation of physical properties to the implementation of practice, the gate-tunable superconductor and insulator phases in such single-material platform are seen as critical serviceable properties in quantum electronic devices. In 2021, both Rodan-Legrain et al.[
4. Other 2D material-based devices
4.1. Transition metal dichalcogenides
Compared with graphene, which possesses linearly dispersed electronic structure and non-gap feature, other 2D materials like TMDs and BP that typically have considerable bandgaps are naturally suitable candidates for applications in electronic and optoelectronic devices. It is known that anisotropy in a crystal structure would be reflected in electronic structure, leading to anisotropic carrier transportation and polarization-sensitive photoresponsivity. But such anisotropic properties can be tuned by band engineering through twisting. For instance, BP, a famous anisotropic 2D material firstly fabricated by Li et al.[
In 2019, Choi et al.[
Similar to optical nonlinearity in TBG including enhanced second- and third-order harmonic generation, TBTMDs have also been discovered controllable matter-light interactions with cavities in strong coupling regimes. The interlayer excitons confined in Moiré potential wells in MoSe2/WS2 heterostructure at the temperature of liquid-nitrogen gave rise to strong nonlinearity, resulting in the density-dependent Moiré polaritons, which could be utilized as tunable arrays of quantum emitters by using long-range light coherence and cavity engineering[
4.2. Black phosphorus
As for other twisted bilayer 2D materials like BP, there have been some valuable experimental data and relevant explanatory theories but most investigations have still been concentrated on theoretical simulations so far, providing meaningful information and bright prospect for approaching studies. It was generally acknowledged that the structural anisotropy along zigzag- (ZZ) and armchair-direction (AC) in BP was regarded as the origin of its intriguing optical polarization-sensitive and rectification properties. In other words, if one can control the anisotropy in BP, it will be prospective for BP to act as an ideal candidate for both the controllable photodetector and diode. Fortunately, single twisting without heteroatomic doping or other complex treatment could achieve the goal, verifying the effectiveness of the new freedom level, twist angle, to 2D systems.
In 2016, Cao et al.[
One year later, in 2017, Xin et al.[
In 2018, more attractive properties in aforementioned cross-stacked BP junction (CBPJ) were discovered by the same group experimentally[
In 2020, from the perspective of electron tunneling, cross-stacked TBBP was modeled to form a monolayer-bilayer-monolayer nanojunction and theoretically simulated rectifying behavior by Shukla et al.[
In the same year, versatile twist-angle were considered in theoretical calculation in TBBP to unveil deeper mechanisms of the rotated angular modulated properties. Yu et al.[
4.3. Hexagonal boron nitride
Hexagonal boron nitride (h-BN), which has the same crystal structure as graphene but consists of boron and nitrogen atoms, is a famous and preferable insulator with atomic thickness due to the relatively low covalency between atoms and thus the large bandgap. The analogous 50-fold enhancement of second harmonic generation observed in twisted bilayer h-BN evidenced that h-BN[
Hence, benefiting from a smaller scale of the Moiré pattern, the twisted bilayer h-BN required smaller coercive electric field than the parallel-stacked bilayer h-BN because not only the migration distance of the domain walls in the former one was much shorter but also the domain wall of the latter one was easy to be pinned by pinning centers, leading to characteristic features in resistance curves of above two cases, respectively (Fig. 12(i)). At last, its outstanding stability of polarization at room temperature, shown in Fig. 12(j), made it prospective in practical utilization of atomic-thick controllable ferroelectric devices.
In summary, the following Table 2 exhibits primary 2D twist-angle superlattices aforementioned in the article with their twist-angle and several electrical and/or optoelectrical property parameters. From both the number of relevant research papers and the quality of 2D twist-system-based devices, it was suggested that making twist-angle 2D superlattices into (opto)electronic application is still in babyhood. Since twisting needs no heteroatoms introducing which guarantees the purity and simplicity of the platform, there will be increasing studies on this field, further promoting the development of applications in (opto)electronics.
5. Conclusion and outlooks
In this article, we elucidated the phylogeny of twist-angle 2D superlattices, especially the feats focusing on the magic-angle twisted bilayer graphene. Basically, the novel and intriguing properties of twisted 2D superlattices can be attributed to the twist-angle-dependent electronic structures with the formation of a Moiré pattern. So far, it is generally accepted that the significant enhancement of photoresponsivity in TBG is correlated with the emergence of van Hove singularities in DOS due to the merging of two Dirac cones of top and bottom sheets, while the distinct enhancement of a photoluminescent response in TBTMDs is caused by the recombination of interlayer excitons stemming from the adjustable interlayer coupling. Besides, twist-induced new transition routes and could significantly enhance the matter-light interaction provide the opportunity to promote the applications in optical nonlinearity of twisted 2D systems.
It is notable that there is a so-called magic angle in the twisted bilayer graphene system, at which the Dirac cones around the Fermi level are flattened and the density of states of carriers is localized, resulting in strong correlated states and thus a series of intriguing phenomenon. Combining with the external electrostatic field, MATBG has been made a Josephson junction and a single electron transistor by tuning different regimes into different phases. This inspires later studies on analogous twisted 2D superlattices like TBTMDs which performed similar flat bands and correlated states under certain conditions while the similar honeycomb structure in h-BN certified the possibility for the realization of controllable ferroelectricity through rotational stacking.
Considering the difficulty in fabrication and instability, predicted properties of other twisted 2D systems, such as BP, still lack experimental realization, even though outstanding tunable anisotropy, rectification and polarization-sensitivity have been theoretically proved. Hence, it calls for further investigation on the verification of the intriguing predictions. On the other hand, the development of an optoelectronic device based on TBTMDs are obviously slower than TBG, not only because of the more intricating elemental constituents and crystal structure, but also due to the inefficient band engineering through twisting. Theoretically, however, TBTMDs should possess similar attractive properties and applications as TBG, hence research both on simulation and experiment are required, with the help of understanding the deeper mechanisms for TBG-analogous features in TBTMDs.
We believe more surprising findings are on the way and these outstanding feats on twist-angle 2D superlattices are just a commencement. Owing to the new freedom in tuning the (opto)electronic properties, the twisted 2D systems are expected to become critical candidates for controllable microelectronic and optoelectronic devices.
Acknowledgements
This work was financially supported by the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB43000000), and the CAS-JSPS Cooperative Research Project (No. GJHZ2021131).
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