Birefringence after Tellurium Nanosheets and Copper Intercalation

Abstract

Symmetry-deficient two-dimensional (2D) layered materials induce a highly anisotropic optical response due to the anisotropy in their crystal structure, facilitating their application in polarized nanodevices. Intercalation is a new way to tune the optoelectronic properties of materials by inserting guest atoms into layered host materials, and 2D layered structures stacked with van der Waals gaps are a prerequisite for this phase of the technique. In this paper, 2D tellurium nanosheets were synthesized with a hydrothermal method, and copper atoms were inserted with a wet chemical method. The widening of the crystal plane spacing proves the introduction of copper atoms, and polarization−related second-harmonic-generation (SHG) studies reveal the intrinsic anisotropic modes of the two samples, and birefringent properties are found with polarizing light microscopy. We further investigated the electrical properties of the samples, and the embedding of the copper atoms caused the samples to exhibit higher currents, but their devices lost the gate control effect.

1. Introduction

Since the discovery of graphene, 2D layered materials have become popular in materials science, optics, and electronics due to their structural properties [1,2]. Unlike the isotropy of graphene, black phosphorus crystals have significant structural differences in the in−plane armchair and zigzag directions, resulting in different bond lengths and bond angles between atoms, which show anisotropy in the crystal structure [3,4]. With the rapid progress in the study of symmetry-deficient materials, more and more 2D materials with anisotropy have been found. The differences in structural properties make them exhibit high anisotropy in optical properties, such as light polarization and birefringence phenomena. These anisotropic optical responses open up possibilities for novel optical devices with polarization-dependent 2D layering [5,6,7,8,9].

Research on 2D layered semiconductors continues to progress with the emergence of new compounds, physical phenomena, and technological innovations. The intercalation technique is used to create layered embedded materials with unique physicochemical properties by filling the van der Waals gap of the host material with guest atoms. These intercalated materials exhibit unique modulation in electrical, optical transmittance; electrical conductivity; thermal conductivity; and energy storage properties [10,11,12]. For example, the embedding and extraction of lithium ions in layered graphite during charging and discharging [13]. Intercalation of compounds in 2D graphite exhibits intriguing superconducting properties [14]. For lithium−ion intercalation in molybdenum disulfide, the embedding of lithium ions will cause the molybdenum disulfide sheet to become transparent [15]. Another example in this regard is the insertion of zero−valent copper atoms in semiconductors, where the insertion of copper produces some interesting effects, such as an increase in conductivity when copper is inserted into bismuth triselenide, and an enhancement of the infrared spectral response when copper is inserted into molybdenum disulfide [16,17,18].

Tellurium has a unique chiral chain structure, which is formed with covalent bonding of a tellurium atom to two neighboring tellurium atoms. Tellurium nanostructures, because of their anisotropy, first grow in the c−axis direction (highlighting their one-dimensional structure) and then grow laterally according to <120> (highlighting their two-dimensional structure), and these laminar structures extend <100> to form2D tellurium nanostructures that are stacked together with van der Waals forces, and this van der Waals stacking makes the engineering of the interpolations possible [19,20,21]. Previous studies have shown that tellurium, as a p-type semiconductor, has a small indirect band gap in the infrared region and exhibits excellent carrier mobility due to spin–orbit coupling. It also exhibits other attractive properties such as thermoelectricity, piezoelectricity, and conductivity, and is widely utilized in photodetectors and fast devices [22,23,24,25]. In this study, we hydrothermally synthesized tellurium nanosheets embedded with copper atoms using wet chemical methods. The optical and electrical properties of tellurium nanosheets were investigated. It was found that tellurium nanosheets have good mobility and intrinsic anisotropy, and polarization microscopy also revealed their birefringent properties. The embedding of copper atoms in the tellurium nanosheets caused them to lose some of their semiconductor properties, and the devices were insensitive to changes in gate pressure. In addition, the tellurium–copper samples showed similar anisotropy and birefringence.

2. Methods

2.1. Chemicals

Sodium tellurite (Na₂CO₃) and polyvinylpyrrolidone (PVP; average molecular weight, 58,000) were purchased from Aladdin Reagent Co. (Shanghai, China). Hydrazine hydrate (N₂H₄-H₂O) and ethylenediamine (EDA) were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China); tetrakisacetonitrile copper hexafluorophosphate was purchased from Anhui Zesheng Technology Co. (Anhui, China). The n-butanol was purchased from Shanghai McLean Biochemical Technology Co. (Shanghai, China).

2.2. Synthesis of Tellurium Nanosheets

The two-dimensional tellurium nanosheets were synthesized with a hydrothermal method; firstly, 0.5 mmol of sodium tellurite and a certain amount of polyvinylpyrrolidone were weighed and put into the inner liner of the reactor of polytetrafluoroethylene, and 33 mL of distilled water was added and stirred with a magnetic stirrer until it was clarified and transparent. Then, ethylenediamine and hydrazine hydrate were added and stirred until clear, and the reactor liner was put into the reactor to be sealed. The reactor was put into an oven and kept at 180 °C for 24 h. The oven was closed and the reactor was cooled to room temperature. The reactor was opened to obtain a silver−white product, which was centrifuged at 10,000 r.p.m. for 10 min, and then washed repeatedly with distilled water and ethylene glycol (to wash away particulate impurities in the product).

2.3. Embedding of Copper Atoms

Copper atoms were intercalated into 2D tellurium nanosheets with a wet chemical method; 0.003 g of tetrakisacetonitrile copper hexafluorophosphate was added to 3 mL of n-butanol and stirred for 8 h in a glass vial until it was clarified and clear, and then the tellurium nanosheets were added and stirred until homogeneous. The glass vial was kept in an oven at 55 °C for 6 h. The silver−white product was then obtained with centrifugation and washed repeatedly with distilled water and ethylene glycol.

3. Results and Discussion

We synthesized 2D tellurium nanosheets with a hydrothermal method using distilled water as a solvent and polyvinylpyrrolidone in the presence of hydrazine hydrate to reduce sodium tellurite under alkaline conditions at a temperature of 180 °C (see Synthesis Methods for detailed synthesis path). Figure 1a exhibits the Te crystal structure diagram, where neighboring Te atoms are linked with covalent bonds to form a helical chain−like structure, and these helical chains will be stacked together with van der Waals forces. When viewed from the x−axis, these layers are stacked together with van der Waals to form a three−dimensional structure. The Te nanosheets were intercalated with copper atoms with wet chemistry using tetrakisacetonitrile copper hexafluorophosphate as the copper source in the presence of n-alcohol (see synthetic methods for detailed synthetic pathways). The superposition of the XRD data of 2D tellurium nanosheets and Te-Cu shows (Figure 1b) that the XRD of the Te nanosheet samples is in good agreement with the standard card, and their narrow diffraction peaks exhibit a better crystallinity of the synthesized products. The XRD picture of the sample after the embedding of the Cu atoms shows that it matches the peak position of the unembedded sample and no new structural phases were created. No diffraction peaks were observed for any bulk metal insertion of atomic copper, confirming the absence of metal precipitation on the nanosheets. The inset is a partially enlarged image of the XRD picture. There is a slight left shift of the peak position of the sample after the copper insertion, showing the enlargement of the sample lattice. At the same time, the intensity of the diffraction peaks changes with the insertion of the material.

Figure 2a shows the elemental distribution mapping (EDS mapping) of the SEM of the Te-Cu sample and its corresponding energy dispersive spectroscopy collection. The Te-Cu sample clearly shows the presence of copper atoms (red). HRTEM images of the tellurium nanosheets and Te-Cu samples are demonstrated in Figure 2b,c. We measured the average value of the stripe spacing of the samples and found that the lattice spacing of the samples with embedded Cu atoms was larger than that of the untreated samples, and their stripe spacing expands from 0.222 nm to 0.23 nm, which is a 0.008 nm increase, and 3.6% of the spacing expansion is significant. EDS mapping and HRTEM images demonstrate the insertion of copper atoms in the 2D Te nanosheets. The insertion of copper causes structural changes in the Te nanosheets and will play an important role in the alteration of tellurium’s optical and electrical properties. Figure 2d shows the Raman spectra of the uninserted and inserted samples; the inserted samples do not show new vibrational modes, and the picture shows the frequency shift from the initial Raman vibrational modes, with the insertion of copper atoms; the optical phonon vibrations harden and the frequency shifts to the right by 2 cm⁻¹, which is also observed in the other inserted systems [26,27].

The in−plane optical anisotropy of samples was studied and the phenomenon of birefringence was revealed. SHG is a second-order nonlinear optical phenomenon in which two photons of the same frequency interact with a nonlinear material and merge into a new photon of twice the frequency. Structures with non−central symmetry/no antisymmetry are the only ones in which the SHG phenomenon can be observed. SGH is widely used to study crystal orientation, symmetry, and stacking of layered samples of materials [28,29]. Tellurium nanostructures consist of chiral chains and naturally belong to symmetry−deficient crystals [30,31]. We deposited Te and Te-Cu nanosheet samples on silicon wafers to study their crystal anisotropy with SHG polarization. A 1064 nm laser was used as the light source, and the incident light was extended in the direction of the c−axis of the Te and Te-Cu samples, and the samples were rotated for 1 week in 15° steps. Figure 3a,b exhibits the polarization diagrams of the samples in response to the SHG signal at different angles. The two samples manifest similar in-plane intrinsic anisotropy. Polarized light microscopy is an effective means of studying the phenomenon of birefringence in materials. When a monorefractive substance is placed on the stage, no light is visible in the microscope because the two polarizers are perpendicular, whereas when a birefractive substance is placed, the light is deflected as it passes through such a substance, and therefore such an object can be detected by rotating the stage. Figure 3c shows the polarized microscope picture of two samples; at 0°, because the two vibration directions of the sample and the vibration direction of the two polarizing mirrors are the same, it becomes an extinction state; when the sample is rotated to 45°, the polarized light reaches to the object, and the decomposed part of the light can pass through the polarizing mirror, and the image shows the brightest state, and the image of the sample is changed to bright and dark with the rotating angle at a period of 90°, and both samples show the birefringent phenomenon; the sample with copper atoms inserted appears a little brighter at 45°.

We investigated the electrical properties of the samples, especially as field effect transistors. We fabricated Te and Te-Cu diode devices with dry transfer. The samples were homogeneously dispersed in an ethanol solution and then deposited on SiO₂/Si sheets. We fabricated gold electrodes with a regular thickness of 100 nm with the vapor deposition process, and the devices were made with dry transfer to both ends of the material. Figure 4a shows the optical microscope photograph of the field effect transistor, and the electrode has a good contact surface with the channel material, and the gold electrode uniformly wraps the tellurium nanosheets. The gold electrodes at both ends are used as the source and drain of the device, the sample is made as the channel material, silicon dioxide is used as the insulating layer, and silicon is used as the gate. Figure 4b shows the I−V curves of the Te and Te-Cu field effect transistors, and both devices exhibit good ohmic linearity. The insertion of copper introduces more conducting atoms into the Te nanosheets and the Te-Cu device exhibits a higher current. Figure 4c exhibits the output curves of the Te field effect transistor, and it can be seen that both devices have good ohmic linearity contacts. Figure 4d shows the transfer curve of the tellurium field effect transistor, and the device exhibits p−type carrier transport behavior. We investigate the mobility of the devices by using the following equation [32]:

$$\mu =\frac{d{I}_d}{d{V}_g} * \frac{L}{D{V}_d{C}_i}$$

where I_d is the source current, V_g is the gate voltage, V_d is the drain voltage, L and D are the length and width of the channel material, the aspect ratio of the sample is 3.54, and C_i = 1.09$\times$10⁻⁸ Fm⁻² is the capacitance of the gate oxide. The tellurium field effect transistors exhibit mobilities of 260, 254, 267, 272, 277, and 286 cm²V⁻¹s⁻¹ at room temperature for V_d −0.01, 0.11, 0.21, 0.31, 0.41, and 0.51 V. Figure 4e,f exhibit the output and transfer curves of the Te-Cu ’s field effect transistor, and it can be seen that the embedding of the Cu atoms in the source−drain current of the sample is almost unaffected by the gate voltage. For field effect transistor devices, the loss of gate control is fatal. The embedding of the copper atoms causes the tellurium nanosheets to lose some of their semiconductor properties and become more of a resistive material.

4. Conclusions

We synthesized Te nanosheets with a hydrothermal method, and the embedding of copper atoms was carried out with a wet chemical method. XRD images show that the sample exhibits good crystallinity. Structurally, the embedding of copper atoms led to the widening of the interlayer spacing of the Te nanosheets by 3.6%, which was accompanied by the hardening of Raman modes. Both Te and Te-Cu samples exhibit intrinsic anisotropy with polarized SHG images, and polarization microscopy images reveal the birefringent properties of the samples. The presence of Cu atoms caused a change in the electrical properties of the Te samples, and the Te-Cu samples exhibited higher currents and insensitivity to the gate voltage.