The photocurrent response of the cr3 junction device. (A) Schematic of atomic force microscopy (AFM) ground state (↑↓↑↓) four-layer cr3 junction device with graphene contact and hBN package at the top and bottom. (B) I-V curve (black curve) of four layers of cr3 junction (D2) under 1 μW 1.96 ev laser excitation. The illustration is a magnified view of the photoelectric stream generated at zero bias Iph and open voltage Voc. (C) differential reflectance (ΔR/R; black spots) and photoelectric current (Iph; As a function of the photon energy of the three-layer (3L) CrI3 at −2 t. The photocurrent is measured by a layer-three (3L) CrI3 junction device (D1) with an optical power of 10 μW. (D) Optical microscopy image of a 3L cr3 junction device (D1). Scale bar, 5 μm. (E and F) Spatial plots of the photocurrent and RMCD signals measured by the same device at 0 T optical power of 1 μW. Scale bar, 5 μm. Source: Science Advances, 10.1126/sciad .abg8094
In a new report published in the journal Science Advances, Tiancheng Song and a team of researchers in the Department of Physics at the University of Washington, usa, as well as materials and nanoarchitectures in Japan and China, detailed the spin photovoltaic effects in two-dimensional (2D) magnetic chromium triiodide (CrI3) sandwiched in van der Waals heterostructures in graphene contact points. Abstract The concept of van der Waals crystals and their heterogeneous structures is a hotspot in materials science, applied physics and optoelectronics, aiming to explore the photoelectric properties in the two-dimensional (2D) range. The integrated two-dimensional magnet can realize two-dimensional spin photoelectric with controllable spin freedom. The photocurrent of CrI3 shows a clear dependence on optical helix, which song et al. adjusted by changing the magnetic state and photon energy. The study highlights the potential to study the phenomenon of optoelectron emergence using engineering magnetic vdW heterostructures.
Spin photoelectric effect
The purpose of spintronics is to regulate the spin freedom of an electronic system to promote new functions. Spin generation and control can provide new opportunities for spintronics to explore new spin photoelectric effects and spin photocurrents. In different heterostructures, the spin photovoltaic effect can be achieved by different mechanisms, where two-dimensional materials such as transition metal dihalogroup are a promising system for spin optoelectronics. The discovery of two-dimensional van der Waals magnets provides a new platform for scientists to study the spin photoelectric effects of thin atomic materials. Among them, chromium triiodide is of interest due to its layered antiferromagnetism (AFM), in which the spin configuration can be regulated by the surrounding magnetic field. The field can switch samples between the AFM ground state and the fully spin-polarized state by a series of flip transitions. The device provides an ideal platform to highlight the spin photoelectric effect at the atomic thin limit.
Spiral dependence of photocurrent in three-layer cr3. (A) The photocurrent measured from the ↑↑↑ state (2t, red dot) and ↓↓ state (−2t, black dot) measured from a three-layer cr3 junction device (D1) with an optical power of 10 μW is a function of the quarter-wavep angle. Vertical arrows indicate line-polarized light. (B) At an optical power of 10 μW, the photocurrent [ΔIph [σ+−σ−]= Iph(σ+)−Iph(σ−)] varies with μ0H. The spiral ΔIph [σ+−σ−]/(Iph(σ+) + Iph(σ−)) illustration shows the schematic of the corresponding magnetic state and circularly polarized light excitation device. (C) RMCD as a function of μ0H of the same device. The illustration shows the corresponding magnetic states and optical microscopy images of the device (D1). Scale bar, 15 μm. Source: Science Advances, 10.1126/sciad .abg8094
The photocurrent response of CrI 3 junction devices and their dependence on magnetic sequence
The researchers developed a vertical heterostructure to study the photocurrent response of CrI3, enabling efficient light detection. The heterostructure consists of an atomically thin cr3 sheet sandwiched between two graphene sheets that act as bias electrodes encapsulate a thin hexagonal boron nitride to prevent degradation. Song et al. further studied the spatial distribution of photocurrents using photocurrent microscopy, and plotted three layers of CrI3 sheets using reflective magnetic circle dichroic method, in which the photocurrent response has a strong dependence on the magnetic order. The team attributed the low and high photocurrent platform states to the antiferromagnetic ground state and the full spin polarization state. Relatively speaking, the photocurrent of the intermediate magnet is low. Photoexcitation produces photoexcitation carriers in the conduction band, where asymmetric extraction of upper and lower graphene electrodes produces the measured photocurrent. Compared with giant magnetoresistance and tunnel magnetoresistance devices, the spin optoelectronic device proposed in this paper provides a new photomagnetic current effect. The resulting huge tunable photomagnetic current is useful for light-driven magnetic sensing and data storage devices.
Dependence of photocurrent on the four-layer CrI3 magnetic sequence. (A) The optical current as a function of the external magnetic field (μ0H), measured by a four-layer (4L) CrI3 junction device (D2), with an optical power of 1 μW. The green (orange) curve corresponds to decreasing (increasing) the magnetic field. (B) RMCD as a function of μ0H of the same device. The illustration shows the corresponding magnetic state and optical microscope image of the device (D2). (C) The tunnel current (It) is measured from the same device under dark conditions at an 80 mV bias as a function of μ0H. The illustration is a schematic of a device with laser excitation and under dark conditions. (D) Four-layer CrI3 Iph-V curves in the AFM ground state (↑↓↑↓, 0 T, black curve) and full spin polarization (↑↑↑↑, 2.5 T, red curve). (E) The size of the photomagnetic current ratio as a bias function extracted from the Iph-V curve in (D). A shade of red indicates the offset range, where | MCph| Tends to infinity. The illustration is a magnified view of the Iph-V curve in (D). Image credit: Science Advances, 10.1126/sciadv.abg8094
Photocurrent mapping in four layers of CrI3. (A) Optical microscopy image of a four-layer CrI3 junction device (D2) (scale bar, 3 μm). Spatial plots of (B) and (C) photocurrent and RMCD signals, measured from the same device at 2.5 T, have optical power of 1 μW (scale bar, 3 μm). Image credit: Science Advances, 10.1126/sciadv.abg8094
Dependence of photocurrent on optical helix and other effects
Song et al. demonstrated the dependence of photocurrent and optical helix by using a three-layer cr3 device with an excitation voltage of 1.96 eV. The resulting unique spin PV effect stems from the spiral dependence of charge transfer excitons in CrI3 coupled to the underlying magnetic order. The helix-dependent absorption of the device reveals the optical selection law of charge transfer between the spin-polarized valence band and the conduction band, resulting in a corresponding speed-dependent spin-photovoltaic effect. Further observations also confirm that the underlying magnetic order is the origin of the charge transfer exciton spiral dependence.
Interaction of magnetic order and photon helix in absorption and photocurrent of 3L cr3. (A) The helix of 3L cr3 in four magnetic states under a selected magnetic field depends on the ΔR/R spectrum. The red (blue) point corresponds to the σ + (σ−) photon helix. The illustration shows an optical microscope image of the corresponding magnetism and three layers of cr3 on the sapphire. (B) A function of the optical current quarter wavelength plate angle ↑↑↑ state (2 T, red dot), ↓↓↓ state (−2 T, black dot) measured by the dotted line in the three selected photon energies (a). (C) ΔR/R spiral difference [(ΔR/R(σ+)−ΔR/R (σ−), curve) and variation in coverage of photoelectric currents (ΔIph[σ+−−σ−) = Iph(σ+)−Iph(σ−),↑↑↑ state (2t, red) and ↓↓ state (−2t, black) is a function of photon energy. Source: Science Advances, 10.1126/sciad .abg8094
foreground
In this way, Song Et al. studied the spin photovoltaic effect in the atomically thin cr3 van der Waals heterostructure. At the same time as the huge photomagnetic current effect, the photocurrent has a clear response to the spin configuration of cr3. Combining spiral-dependent photocurrent and circular polarization-resolved absorption measurements reveals the interaction between spin photocurrent and the underlying exciton, as well as the contribution of magnetic order, photon energy, and spiralness. The two-dimensional photovoltaic device developed in this paper uses the intrinsic magnetic order in several layers of cr3 as a proof of concept. The resulting atom-thin CrI3 forms a prototype two-dimensional magnet for studying the photocurrent generated in vertical junction devices. The device can be adapted to alternative two-dimensional magnets, with potential applications in magnetic sensing and data storage. The basic dynamics of the magnetic sequence coupled charge transfer exciton state can generate a photocurrent to probe the magnetic sequence in cr3 and have a pronounced response to photon energy and spiral. These results highlight the application of photocurrents as a new method for detecting magnetic sequencing, charge transfer exciton states, and magnetic exciton-photon coupling. This method can be used for the study of other two-dimensional magnetic systems, including the dynamics of zigzag antiferromagnetic ordered coupled excitons and the charge transfer process at the graphene interface.