【Introduction】

A new type of 2D transition metal carbides, carbonitrides and nitrides called MXenes have become new candidates for many applications in the field of micro-nanoelectronics, optoelectronics and energy storage. Since its first discovery in 2011, MXenes has received increasing attention due to its unique physical, chemical and mechanical properties (which can be adjusted by different surface functional groups and transition metals). Especially excellent photoelectric properties (including transparency, saturated light absorption and high conductivity) make MXenes capable of fulfilling various roles in photodetectors, such as transparent electrodes, Schottky contacts, light absorbers and plasmons material. Considering that it can be prepared by chemical solution method, MXenes also has great potential for large-scale synthesis, so it is favored by many electronic and photonic device applications.

【Achievement Introduction】

In this review, the team of Professor Wu Jiang from the University of Electronic Science and Technology summarized the latest progress of 2D MXenes-based photodetectors. Although compared with other 2D materials, such applications have not been studied until recently, but MXenes shows considerable potential in the field of low-cost and high-performance light detection. The achievements under the title " Recent in Advances in 2D MXenes for photodetection " published in the internationally renowned journal Adv. Funct. Mater. On .

【Graphic introduction】

Figure 1. Schematic crystal structure and electron microscope image of MXenes

a) Schematic diagram of evolution from MAX phase to MXene

b) MXene M2X crystal model top view (top) and side view (bottom)

c) Crystal structure of functionalized Ti 3 C 2 T x with surface functional groups

df) SEM images show the HF-treated precursors Ti 2 AlC, Ti 3 AlC 2 and Ti 4 AlC 3

g) HAADF-STEM clearly shows the atomic distribution of Mo 2 C crystal

Figure 2. Photoelectric performance of MXenes

a) Transmittance of spin-cast Ti 3 C 2 T x films with different thickness

b) FTIR spectral resolution of Ti 3 C 2 with changed structure and dimensions

c) Transmittance change of Ti 3 C 2 T x films embedded with different cations

d) Absorbance of Ti 3 C 2 T x synthesized by different etching solutions

e) DFT calculation of the absorbance of Ti 3 C 2 sheets in the predicted terahertz (THz) range (0.0012-0.012 eV) for laminated and single layers (inset)

f) Thickness-dependent absorption comparison between MXene and rGO

g) Sheet resistance of Ti 3 C 2 T x thin films with different transmittance (thickness) after different storage treatments

h) Time-resolved transmittance stability of Ti 3 C 2 T x film

i) The relationship between the transmittance of Ti 3 C 2 T x with different surface terminations and the sheet resistance

Figure 3. Photoelectric performance and photodetector performance of Ti 3 C 2 T x / n-Si heterostructure

a) Schematic diagram of heterostructure

b) Transmittance and resistance of different concentrations of Ti 3 C 2 T x colloidal solution

c) Extracted Voc and Jsc as a function of Ti 3 C 2 T x colloid solution concentration as a function of the curve

d) The band structure of the heterogeneous structure in the dark

e) JV characteristics of photodetectors under illumination with varying power intensity

f) Photoresponse dynamics of photodetectors under 405 nm laser irradiation

Figure 4. MXene- based photodetector performance compared to gold-based devices

a) SEM image of MXene-GaAs-MXene junction photodetector

b) Schematic diagram illustrating the alignment band structure of the MXene-GaAs-MXene junction under bias voltage

c, d) Power-dependent photoresponse current of photodetectors based on MXene and Ti / Au when 830 nm laser is irradiated under different bias voltages

e, f) Transient optical response of MXene and Ti / Au-based photodetectors illuminated by 100 fs pulses of 830 nm lasers of different powers

Figure 5. MXenes characterization and environmentally sensitive UV light response performance

a) Schematic diagram describing the partial oxidation-induced Ti 3 C 2 T x -TiO 2 composite material for ultraviolet light detection

b) Transmittance of 16 nm and 38 nm Ti 3 C 2 T x films in the wavelength range of 300–1000 nm

c) Nyquist diagram of 16 nm Ti 3 C 2 T x film in Ar atmosphere (purple circle) and without (red square) UV illumination

d) The time-resolved current of the 16 nm Ti 3 C 2 T x film is directly exposed to air (blue triangle), stored in an Ar atmosphere (red square), and then exposed to air (blue circle)

e) Ultraviolet light response kinetics of 38 nm Ti 3 C 2 T x film in Ar atmosphere and air

f) Ultraviolet light response kinetics of 38 nm Ti 3 C 2 T x thin film, respectively decay in Ar, O 2 , air and H2O vapor atmosphere

Figure 6. Material characterization and photodetector array performance

a) Schematic diagram of perovskite / MXenes photoelectric detector under illumination

b, c) SEM images of Ti 3 C 2 T x and CsPbBr 3 , respectively

d) Time-resolved photocurrent of photodetectors with MXene electrodes of various thicknesses

e) Current-voltage curve of photodetectors with varying distance between electrodes

f) Photocurrent diagram corresponding to the incident light pattern

g) Schematic band structure of perovskite / MXenes photodetector in equilibrium (left) and under light (right)

h) Time-resolved photocurrent of the photodetector array at various bending angles from 180 ° to 60 °

Figure 7. Material characterization of MXene and perovskite and the performance of nanocomposite photodetectors

a, b) SEM and TEM images of the prepared Ti 3 C 2 T x nanosheets and CsPbBr 3 nanocrystals, respectively

c) Light absorption spectrum of CsPbBr3 / Ti 3 C 2 T x nanocomposites with various Ti 3 C 2 T x concentrations

d, e) PL and TRPL spectra of CsPbBr 3 / Ti 3 C 2 T x nanocomposites with different Ti 3 C 2 T x concentrations

f) Time-resolved photocurrent of photodetectors based on CsPbBr 3 / Ti 3 C 2 T x nanocomposites

Figure 8. Plasmons enhance the performance of photodetectors

a) TEM images of several layers of Mo 2 CT x nanosheets and fast Fourier transform (FFT) mode (inset)

b) Digital photo showing flexible photodetector array

c) Spectral wavelength-resolved optical response of Mo 2 CT x nanosheets

d, e) Light response performance related to light intensity, including photocurrent, light response, detection and EQE

f) Comparison of on / off current ratio between five MXenes under 532 nm (0.41 mW cm ^-2 ) and 660 nm (0.39 mW cm ^-2 ) illumination

g) EELS of 58 nm Mo 2 CT x nanosheets, normalized to lateral surface plasmon peaks resolved at 2.45 eV

h) The schematic depicts the process of plasma-assisted hot carrier transport to the Au electrode when the device is biased under light

Figure 9. Photoresponse characteristics of Mo 2 C / MoS 2 photodetector

a, i) Schematic diagram of Mo 2 C / MoS 2 photodetectors with fixed and different grating periods respectively

b) ΔΦ map image showing stacked Mo 2 C / MoS 2 hybrid junction and periodic grating structure

c) Cross-sectional high-resolution TEM (HR-TEM) image, used to identify the Mo 2 C / MoS 2 heterostructure on the SiO 2 substrate

d) Broadband spectrally resolved photodetectors based on pure MoS 2 and Mo 2 C / MoS 2 heterostructures with different grating periods

eh) Compared with different pattern periods of 400-1000 nm, the simulated normalized ECS spectrum of Mo 2 C grating varies with wavelength

j) The photodetectors based on pure MoS 2 and Mo 2 C / MoS 2 heterostructures have different incident wavelength-resolved light responsiveness of the grating period on the same band respectively

k) Under the illumination of different wavelengths, the incident optical density of the device depends on the incident power density

【to sum up】

MXenes exhibit interesting material properties, and the author has reviewed the latest developments in MXene-related photodetectors based on such properties, including photoconductors, self-powered photodetectors, and plasmon-assisted photodetectors. The optical and electrical properties of MXenes (for example, thickness-dependent transmission and absorption, saturable absorption and high conductivity) are ideal for light detection. These characteristics can be adjusted by different etching schemes, surface functional groups and chemical intercalation. Controllable optical and electrical characteristics enable MXenes to play various roles in photodetectors and provide a multi-functional platform for different equipment configurations. In Section 3, MXenes incorporating photodetectors have the following effects: 1) Transparent electrodes, benefiting from good light transmittance and metal conductivity; 2) Schottky contacts, due to adjustable WF from 1.6 to Variation within a wide range of 8.0 eV; 3) Light absorbers, related to potential saturation absorption; 4) Plasmonic photonic materials, produced by metallic behavior. Despite a small amount of exploration of this application, MXenes has shown hope in the field of light inspection. Related research is becoming more and more attractive, especially after expecting to find more interesting MXenes features.

Literature link : Recent Advances in 2D MXenes for Photodetection . Adv. Funct. Mater., 2020 , DOI: 10.1002 / adfm.202000907.

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