SEM image of AuCN deposition on graphene by multiple castings using high concentration (2.4mM). Schematic of electron beam lithography details (line mark and device geometry model pattern). a) Optical image of exfoliated GeSe crystal.
Various 2D materials
From the device application point of view, 2D thin materials are good for tuning the carrier concentration using the electrostatic gate effect.24 In addition, 2D materials are attracting great attention as an alternative to solve the problem of downsizing transistors. metal oxide semiconductor field (MOSFET) based on conventional 3D crystals.
Black phosphorus & orthorhombic IV-VI compounds
The impurity can reduce and the quality of the electrode can be improved by the high vacuum conditions. The density of AuCN was calculated using the percentage of the area covered by AuCN in the SEM image. A careful study of the nanowire morphology showed that the nanowire formation process depends on the growth conditions.
The nanowire morphology with a longer axis in the direction of the atomic chain can be rationalized by the stronger covalent bonding along the chain compared to the weaker van der Waals interaction across the chains. We can rewrite the equation using the definition of the Θ function to convert the integral of the 𝑑𝑘 form to 𝑑𝜀. The Sommerfeld expansion allows to go a step further and transform the integrals into development series in power of 𝑘𝐵2. The target flag must be transferred to the TEM grid to confirm that the optical contrast value and the direction of the actual crystal orientation are coincident.
The disadvantage of using this method is that the electrode can only be deposited in the designed pattern of the shadow mask. In the case of samples from Figure 44a, the direction of the edge for which the reference was set is parallel to the ZZ direction of the crystal.
Various kinds of 1D nano-structural materials
Epitaxial growth of AuCN nanowires on graphene
Various 2D materials are naturally endowed with well-terminated surface, making them highly suitable for van der Waals vertical heterostructures. In this study, we investigated the controlled fabrication and synergetic operation of a 1D/2D van der Waals heterostructure formed with AuCN and graphene.
Experimental methods
- CVD graphene synthesis
- CVD graphene transfer
- CVD graphene metal deopsition by using shadow mask
- Drop cast method for AuCN/graphene heterostructure
In this process, the Cu oxide in the Cu sheet was removed and the Cu is recrystallized. In methane, carbon atoms were seeded from the domain edge of the Cu sheet, and C was attached one by one to synthesize graphene. After etching, the PMMA-coated graphene floating in the aqueous ammonium sulfate solution was transferred to the beaker containing DI water using slide glass.
In this stick state, etch the Cu foil by placing it in ammonium and then rinsing as shown in Figure 11.
TEM, SEM measurement
Epitaxial growth of AuCN nanowires on graphene
Compared with the incubation method, the drop-cast method has the advantages that the density and morphology of AuCN nanowires can be easily controlled. a) Atomic model of AuCN on graphene.
Various density & morphology of AuCN nanowires with different growth conditions
The main mechanism of the high photoresponse observed in AuCN/graphene heterostructures can be explained by the photoclustering effect, which has been previously discussed in other graphene hybrid phototransistors. After placing the sample in the center of the optical microscope, we used a polarizer to make a parallel polarization setup. As can be seen from Figure 46b, d, that the intensity of the B3g Raman peak was maximum when the ZZ direction of the crystal and the polarized laser was 45 degrees to each other.
It was observed that the light is absorbed well through the increase of the current level when the light is illuminated.
Mechanism of AuCN nanowires formation process
Bandgap measurement
Experimental bandgap measurement using UV-visible spectra
Optical absorption measurement and band structure of AuCN. a) Optical transmission of graphene and AuCN/graphene samples. There have been a limited number of research papers70 on the electrical and optical properties of AuCN prior to our work. The band gap of AuCN was determined by measuring the UV-visible absorption spectrum (Cary 5000 UV-Vis-NIR spectrophotometer).
For the conversion of transmittance data into absorption plots, we considered the possible scattering/reflectance changes from rough surface of AuCN-deposited samples.
Theoretical calculation of AuCN bandstructure
Electrical measurement
- Backgated gaphene FET and mobility calculations
- Basic electrical property characterization & doping level change of AuCN/graphene
- Power and photon-energy dependent of light response
- Gate-dependent light response
After the formation of AuCN nanowires on graphene, the AuCN/graphene devices (0.8 mM, 1 drop) showed a change in the transfer curves. Force and Photon Energy Dependent Light Zone. a) I-V curve and (b) transfer curve of AuCN/graphene device under the white light illumination with different power. Therefore, the p-doping from the original electron donation of graphene to AuCN is reduced under light illumination, which is consistent with our experimental observation of the Dirac point shift. a) Photon energy-dependent photoresponsiveness and (b) External quantum efficiency (EQE) of the AuCN/graphene device.
The AuCN/graphene device also showed good switching behavior under on-off light modulation as shown in Figure 29a.
Further study: Universal oriented van der Waals epitaxy of 1D cyanide chains on hexagonal 2D
As shown in Figure 30e,f, the experimentally observed SAED pattern was in good agreement with the simulated diffraction pattern of the AgCN crystal structure for the [100] region axis. The two flakes were stacked so that the twist angle between graphene and h-BN was zero. The simulated diffraction signals from the h-BN/graphene heterostructure with zero twist angle reproduced the observed diffraction pattern (Figure 33d).
These results show that crystal orientation identification using microwires is a reliable method and enables the fabrication of vertical 2D heterostructures with highly precise control of the twist angle.
Conclusion
The GeSe thermoelectric device is fabricated in the form of a commonly used reverse field-effect transistor (FET). For this type of device, good electrical contact is important for investigating the intrinsic properties of GeSe. In the case of GeSe, many studies have been carried out to control the thermoelectric properties through chemical doping or bulk alloying with a size of 132-136 mm.
However, no research on the thermoelectric properties of thin single nm-thick flakes has been reported.
Research background
Seebeck effect
Boltz mann transport and Mott’s formula
We can find the overall electrical conductivity by considering the situation where there is no thermal gradient in this system. We can rewrite the equation using the definition of the function Θ to convert the integral from the form 𝑑𝑘 to the 𝑑𝜀.
Experimental methods
- Mechanical exfoliation
- Identifying the crystal orientation using optical contrast method
- GeSe nanoflake transfer to TEM grid
- Device fabrication by using parylene mask and electrode deposition
- Device fabrication by using e-beam lithography and electrode deposition
After that, we took the optical image of the target flame at every 10 degree interval from 0 to 360 degrees. One is using a parylene mask as a shadow mask and the other is using electron beam lithography. Photograph the image of the manipulator and the parylene mask. a) Schematic of the mask alignment and device manufacturing process.
Attach the pink tape to the manipulator so that the parylene mask is clearly visible as a perforated part.
TEM, AFM, Raman measurements
Characterization of anisotropic property of GeSe
We also performed polarized Raman with flakes confirming the ZZ direction of GeSe crystals using optical contrast method. Initially, we fabricated the device (Figure 46a) to also study the anisotropic electrical properties of GeSe. We fabricated the device as shown in Figure 40a,b to measure the unique thermoelectric properties of GeSe. a) Schematic of GeSe thermoelectric device.
Electrical and thermoelectric properties of GeSe flake. a) Electrical conductivity and (d) power factor as a function of Vg for different temperature.
Surface chaning with air ambient conditions (degradation)
Electrical property of GeSe
Contact issue from thermal evaporator vs sputter deposition
A cross-sectional image of a FET device in thermal vapor deposition. a) Here is a picture of the GeSe device just before the FIB process. First, it was confirmed that the GeSe crystal and the electrode did not touch smoothly and that the impurity was stuck (Fig. 49b,c). In addition, it was confirmed that GeSe changed from a crystalline to an amorphous part when exposed to air.
After sputter deposition, it was confirmed that the contact resistance was greatly reduced because the current level was increased high.
Temperature-dependent electrical property of GeSe single nanoflake
In this context, the threshold voltage which means that the hole doping level increased as the temperature increased (Figure 51b). To explore more about thermionic emission and contact barrier, we calculated the Schottky barrier from our data. 𝐼𝑑𝑠 is the drain current, 𝐴 is the contact area, 𝐴∗ is the Richardson constant, 𝑒 is electron charge, 𝑘𝐵 is the Boltzmann constant, 𝜑𝑏 is the Schottky barrier height, 𝜆 is the ideality factor.
This value was smaller than the known Schottky barrier between Ti and GeSe141 and it was confirmed that electrodeposition using sputtering effectively reduced the contact barrier.
White light response of GeSe single nanoflake
Here the temperature gradient is generated linearly when the length of the electrode used as microheater and thermometer is consistent. We confirmed that the higher the temperature and the greater the amount of negative voltage, the greater the power factor. Effect of stacking order and in-plane strain on the electronic properties of bilayer GeSe.
Structures and negative thermal expansion properties of the one-dimensional cyanides, CuCN, AgCN and AuCN.
Thermoelectric property of GeSe
Working principle of thermoelectric devices
Since the temperature gradient created in GeSe is small, it cannot be measured with an infrared camera, so the temperature is measured through resistance thermometers. Second, we measured the resistance of the thermometer by applying voltage to the micro-heater electrode to obtain the relationship between voltage and temperature gradient across the GeSe nanoflakes. Based on the linear relationship, we calculated the local temperature gradient at the thermometer electrodes under the pressure applied to the heater (Figure 54c).
The linear relationship obtained in Figure 54c allowed us to read a stress buildup as the temperature gradient emerged, and the slope value is the Seebeck coefficient.
Seebeck coefficient optimizing by using gate voltage
When this was done, the Seebeck coefficient was largest when the gate voltage was 80 V, and smallest when the gate voltage was −40 V. As shown in Figure 50a, GeSe is a typical p-type, so as the gate voltage, the Fermi level increases and the hole carrier decreases, and as the voltage decreases, the Fermi level decreases and the hole carrier increases. Finally, we measured the voltage rise while applying joule heating to the micro heater, then we can get the Seebeck coefficient (-ΔV/ΔT).
As the gate voltage was increased, the carrier concentration decreased and the S value increased.
Conclusion
Experimental identification of critical conditions for drastically improving the thermoelectric power factor of two-dimensional layered materials. Direct growth of monolayer and few-layer MoS2 on h-BN with preferred relative rotation angles. Epitaxial growth of molecular crystals on van der Waals substrates for high performance organic electronics.
