In-Situ Measurement Of Electrical Conductivity In Diamond Anvil Cell

With the development of science and technology, more and more in situ experiments can be performed in diamond anvil cells (DACs), such as synchrotron X-ray diffraction, laser Raman scattering, photoluminescence, optical absorption and so on. These increasingly mature techniques have led to a great improvement in the research of high pressure physics. However, also as an important measurement technique, the development of electrical resistance measurement in DAC is slow, which is due to the problem of insulating layer and the difficulty of probe arrangement in tiny sample chamber. In this thesis, for solving above technical problems, electrical probes and insulating layer are integrated directly onto the diamond anvil. By using this experimental system, in-situ electrical measurements are performed under high pressure on the samples of ZnS, ZnO and C3N4 with the graphitic structure (g-C₃N₄). For the electrical resistance or resistivity measurements in DAC, the probe arrangements mainly contain two methods, manual arrangement and integration method by using thin film sputtering and photolithographying techniques. The way of probe arrangement adopted in our electrical measurement is the second method. It shows some outstanding advantages as following, (1) the sputtered probes can be designed with various regular shapes by photolithographying and exactly placed at a desirable place on the diamond culet; (2) the probes can remain unchanged under high pressure. These advantages make the accurate measurement of resistivity in DAC possible. We use quasi four-probe arrangement in our experiment. This arrangement can eliminate most of contact resistance between the sample and the probes. In our microcircuit we chose molybdenum (Mo) for the conductor and alumina (Al2O3) for the insulator and protective materials in the resistivity measurement. Mo and Al2O3 have similar large bulk moduli (Mo,230 GPa, Al2O3 ,240 GPa). This minimizes the plastic flow caused by compression along the axial direction and by shear stress along the radial direction, and thus ensures the stability of the circuit structure. Also, they have stable crystal structures to over megabars (Mo, 210 GPa and Al2O3, 175 GPa), which eliminates the error and complexity introduced thereby. In addition, the high hardness of Al2O3 HB≈1500 kgf/mm2), nonconduction to very high pressure and good wearability are additional properties for better insulation and protection performance in a resistance measurement. The fabrications of Mo thin film and Al2O3 layer are carried out in a radio frequency sputtering system. Photolithographic technique is used to pattern the Mo thin film into quasi four-probe electrodes. With the designed diamond anvil, we perform in-situ high-pressure electrical resistance measurements on nanocrystalline ZnS, bulk ZnS, ZnO and g-C3N4. Electron transport is very sensitive to lattice. Under external pressure the structure of matter will be changed, which results in the change of electron transport accordingly. In the measurement of nanocrystalline ZnS, it is found that a significant drop (about six order of magnitude) of resistance can be observed between 21 GPa and 23 GPa in compression process. The pressure at which the resistance drops suddenly is related to the phase transition from zinc-blend structure to rock-salt structure in ZnS. Resistance variation during decompression is also studied. The reversible behavior of this phase transition can be reflected by the resistance measurements. But the transition pressure in the decompression process is apparently lower than that in the compression process. This hysteresis relates to the activation energy difference of ZnS between the zinc-blend and the rock-salt structure. Similar phenomena can be observed in bulk ZnS sample. But the transition pressure of bulk ZnS sample is apparently lower. The higher transition pressure in nanocrystalline ZnS sample is caused by nanometer scale effect. Additionally, the hysteresis width of bulk ZnS is narrower than that of nanocrystalline ZnS ZnO is also an important II-VI semiconductor. Electrical resistance measurement on bulk ZnO indicates that the phase transition occurred in ZnO is unreversible, its high pressure phase can remain metastable at ambient pressure.