液態金屬氧化物在近來的研究中被受到重視有以下原因，其一是為發展出製程簡便的方法可單獨取出大面積連續薄膜(mm~cm)進行大量生產應用，其二是由於其原生氧化層厚度受限於Carberra-Mott potential，在z軸方向僅有數埃到幾奈米厚，在做成電子元件時因為電子通道被限制，因此有很好的電子遷移率，其三為可重複性，在轉印完成後回收透過酸洗或鹼洗去除氧化層後，可做下一次使用，對金屬原料而言整體消耗量極低，相對於傳統用sputter靶材鍍膜或MBE生長材料，在近來強調環保及永續使用材料來說，此方法快速且可以省下更多的成本，有很好的商用潛力。雖說液態金屬氧化物有上述很好的應用前景，但是二維材料相比於塊材及一維的奈米線有更嚴格的thermal budget限制，製程和結晶條件更加嚴苛，其電性質和塊材相比有許多未知之處，因此本研究將分成二維磷酸鎵的壓電材料合成分析和元件性質這兩大部分探討。
從一系列參數的實驗結果可得知如果直接將非晶態的二維氧化鎵直接做反應並後退火結晶其鍵結並不是單純的磷酸鎵薄膜，而是複合多種鍵結，導致結果的結晶性不佳，因此我們提出先在大氣退火強制氧化成氧化鎵再做反應的製程想法，再後退火處理，可以發現不管是鍵結、XRD繞射、還是TEM的晶面分析，都顯示其具高度結晶性，結晶取向為[01 ̅0]的面內結晶。最後我們使用這個製程參數做電性量測、並利用PDMS封裝製作成壓電電子元件，量測其輸出，得到其在5N開始就會有很明顯的壓電響應，在50N會出現最大輸出電壓14.52mV、-39.93nA的壓電輸出電流，並發現二維GaPO4也具有壓阻現象，由於現在輸出可能會受下層二氧化矽基板的缺陷影響，未來將會透過二次薄膜轉印的方式再轉移在PDMS上製作成d11的壓電電子元件。

We investigate various parameters for synthesizing GaPO4, it can be concluded that directly reacting and annealing amorphous two-dimensional gallium oxide does not yield a high crystallinity gallium phosphate film. Instead, it results in a complex mixture of compounds, leading to poor crystallinity. Therefore, we propose a process that involves pre-oxidizing the gallium to form gallium oxide in the atmosphere before the reaction. After pre-oxidation, the subsequent reaction and annealing process demonstrate that the bonding, X-ray diffraction (XRD), and transmission electron microscopy (TEM) analysis all indicate high crystallinity with in-plane orientation of [01 ̅0]. Finally, using this process parameter, we conducted basic electrical measurements and encapsulated the material in PDMS to fabricate a piezoelectric electronic device. We observed a significant piezoelectric response at 5N, and a maximum d12 output voltage of 14.52 mV and a piezoelectric output current of 39.93nA at 50N. Additionally, we discovered that two-dimensional GaPO4 exhibits piezoresistive properties. Since the current output may be affected by defects in the underlying silicon dioxide substrate, future work will involve secondary film transfer to PDMS to fabricate a d11-mode piezoelectric electronic device.

1. Du, B., et al., Liquid Ga–In–Sn alloy printing of GaInSnO ultrathin semiconductor films and controllable performance field effect transistors. ACS Applied Electronic Materials, 2023. 5(3): p. 1879-1890.
2. Chang, C.-M., C.-Y. Wu, and C.-W. Huang, Photoresponse of large-area atomically thin β-Ga2O3 and GaN materials via liquid metal print synthesized. Applied Surface Science, 2024. 649: p. 159131.
3. Xu, Y., et al., Squeeze-printing ultrathin 2D gallium oxide out of liquid metal for forming-free neuromorphic memristors. ACS Applied Materials & Interfaces, 2023. 15(21): p. 25831-25837.
4. Moon, S., et al., 2D Amorphous GaOX Gate Dielectric for β-Ga2O3 Field-Effect Transistors. ACS Applied Materials & Interfaces, 2023. 15(31): p. 37687-37695.
5. Wurdack, M., et al., Ultrathin Ga2O3 glass: a large‐scale passivation and protection material for monolayer WS2. Advanced Materials, 2021. 33(3): p. 2005732.
6. Farghali, M., et al., Strategies to save energy in the context of the energy crisis: a review. Environmental Chemistry Letters, 2023. 21(4): p. 2003-2039.
7. Malhotra, J., et al. Energy Harvesting Applications using Piezoelectric Sensors. in Proceedings of the International Conference on Recent Advances in Computational Techniques (IC-RACT). 2020.
8. Jiang, H., et al., Two‐dimensional materials: From mechanical properties to flexible mechanical sensors. InfoMat, 2020. 2(6): p. 1077-1094.
9. Zhang, H., Ultrathin two-dimensional nanomaterials. ACS nano, 2015. 9(10): p. 9451-9469.
10. Zavabeti, A., et al., A liquid metal reaction environment for the room-temperature synthesis of atomically thin metal oxides. Science, 2017. 358(6361): p. 332-335.
11. Syed, N., et al., Printing two-dimensional gallium phosphate out of liquid metal. Nature communications, 2018. 9(1): p. 3618.
12. Geim, A.K. and I.V. Grigorieva, Van der Waals heterostructures. Nature, 2013. 499(7459): p. 419-425.
13. Anasori, B. and Û.G. Gogotsi, 2D metal carbides and nitrides (MXenes). Vol. 2549. 2019: Springer.
14. Novoselov, K.S., et al., Electric field effect in atomically thin carbon films. science, 2004. 306(5696): p. 666-669.
15. Li, J., et al., Wafer-scale single-crystal monolayer graphene grown on sapphire substrate. Nature Materials, 2022. 21(7): p. 740-747.
16. Castellanos-Gomez, A., et al., Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping. 2D Materials, 2014. 1(1): p. 011002.
17. Suk, J.W., et al., Transfer of CVD-grown monolayer graphene onto arbitrary substrates. ACS nano, 2011. 5(9): p. 6916-6924.
18. Xu, C. and W. Gao, Pilling-Bedworth ratio for oxidation of alloys. Material Research Innovations, 2000. 3: p. 231-235.
19. Fehlner, F.P. and N.F. Mott, Low-temperature oxidation. Oxidation of metals, 1970. 2(1): p. 59-99.
20. vlad, A. KINETICS OF OXIDE GROWTH ON METAL SURFACES. 2005; Available from: https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=14d2b13effb3da21dbd703660a210bb420c94aec.
21. Atkinson, A., Transport processes during the growth of oxide films at elevated temperature. Reviews of Modern Physics, 1985. 57(2): p. 437.
22. Cabrera, N. and N.F. Mott, Theory of the oxidation of metals. Reports on progress in physics, 1949. 12(1): p. 163.
23. Fu, Q. and T. Wagner, Interaction of nanostructured metal overlayers with oxide surfaces. Surface science reports, 2007. 62(11): p. 431-498.
24. Goff, A., et al., An exploration into two-dimensional metal oxides, and other 2D materials, synthesised via liquid metal printing and transfer techniques. Dalton Transactions, 2021. 50(22): p. 7513-7526.
25. Plech, A., et al., In situ x-ray reflectivity study of the oxidation kinetics of liquid gallium and the liquid alloy. Journal of Physics: Condensed Matter, 1998. 10(5): p. 971.
26. Regan, M., et al., X-ray study of the oxidation of liquid-gallium surfaces. Physical Review B, 1997. 55(16): p. 10786.
27. Daeneke, T., et al., Liquid metals: fundamentals and applications in chemistry. Chemical Society Reviews, 2018. 47(11): p. 4073-4111.
28. Liu, Q., et al., Room-temperature preparation of large-area transparent two-dimensional ZnO-doped Ga2O3 nanostructure-based layers: implications for optoelectronic nanodevices. ACS Applied Nano Materials, 2023. 6(4): p. 3027-3035.
29. Li, Q., et al., Liquid metal gallium-based printing of Cu-doped p-type Ga2O3 semiconductor and Ga2O3 homojunction diodes. Applied Physics Reviews, 2023. 10(1).
30. Kochat, V., et al., Atomically thin gallium layers from solid-melt exfoliation. Science advances, 2018. 4(3): p. e1701373.
31. Cooke, J., et al., Synthesis and Characterization of Large‐Area Nanometer‐Thin β‐Ga2O3 Films from Oxide Printing of Liquid Metal Gallium. physica status solidi (a), 2020. 217(10): p. 1901007.
32. Li, J., et al., Template approach to large-area non-layered Ga-group two-dimensional crystals from printed skin of liquid gallium. Chemistry of Materials, 2021. 33(12): p. 4568-4577.
33. Du, Y., et al., Liquid‐Metal‐Assisted Synthesis of Patterned GaN Thin Films for High‐Performance UV Photodetectors Array. Small Methods, 2024. 8(2): p. 2300175.
34. Ma, R., Y. Zhou, and J. Liu, Erasing and Correction of Liquid Metal Printed Electronics Made of Gallium Alloy Ink from the Substrate. arXiv preprint arXiv:1706.01457, 2017.
35. Park, J., et al. A Review on Recent Advances in Piezoelectric Ceramic 3D Printing. in Actuators. 2023. MDPI.
36. Cui, C., et al., Two-dimensional materials with piezoelectric and ferroelectric functionalities. npj 2D Materials and Applications, 2018. 2(1): p. 18.
37. Hinchet, R., et al., Piezoelectric properties in two-dimensional materials: Simulations and experiments. Materials Today, 2018. 21(6): p. 611-630.
38. Zelisko, M., et al., Anomalous piezoelectricity in two-dimensional graphene nitride nanosheets. Nature communications, 2014. 5(1): p. 4284.
39. Yang, P.-K., et al., Tin disulfide piezoelectric nanogenerators for biomechanical energy harvesting and intelligent human-robot interface applications. Nano Energy, 2020. 75: p. 104879.
40. Dai, M., et al., Enhanced piezoelectric effect derived from grain boundary in MoS2 monolayers. Nano letters, 2019. 20(1): p. 201-207.
41. Wu, W., et al., Piezoelectricity of single-atomic-layer MoS2 for energy conversion and piezotronics. Nature, 2014. 514(7523): p. 470-474.
42. Ma, W., et al., A novel multi-flaw MoS 2 nanosheet piezocatalyst with superhigh degradation efficiency for ciprofloxacin. Environmental Science: Nano, 2018. 5(12): p. 2876-2887.
43. Guo, J., et al., Enhanced NO2 gas sensing of a single-layer MoS2 by photogating and piezo-phototronic effects. Science Bulletin, 2019. 64(2): p. 128-135.
44. Tak, B.R., et al., Recent advances in the growth of gallium oxide thin films employing various growth techniques—a review. Journal of Physics D: Applied Physics, 2021. 54(45): p. 453002.
45. Higashiwaki, M., et al., Recent progress in Ga2O3 power devices. Semiconductor Science and Technology, 2016. 31(3): p. 034001.
46. Ahmadi, E. and Y. Oshima, Materials issues and devices of α-and β-Ga2O3. Journal of Applied Physics, 2019. 126(16).
47. Armand, P., et al., Flux-grown piezoelectric materials: application to α-quartz analogues. Crystals, 2014. 4(2): p. 168-189.
48. Jacobs, K., et al., Structural phase transformations in crystalline gallium orthophosphate. Journal of Solid State Chemistry, 2000. 149(1): p. 180-188.
49. Shvanskii, E., et al. Flux growth of gallium orthophosphate crystals. in Annales de Chimie-Science des Matériaux. 2006.
50. 林景崎, 整合式表面分析系統簡介. 科儀新知, 2003. 25(2): p. 95-99.
51. 寺島雅弘, et al., LEIPS および UPS を用いた有機 EL 材料のバンド構造評価. Journal of Surface Analysis, 2020. 27(1): p. 34-44.
52. Mizokawa, Y., O. Komoda, and S. Miyase, Long-time air oxidation and oxide-substrate reactions on GaSb, GaAs and GaP at room temperature studied by X-ray photoelectron spectroscopy. Thin Solid Films, 1988. 156(1): p. 127-143.
53. Nishitani, R., et al., An XPS analysis of thermally grown oxide film on GaP. Japanese Journal of Applied Physics, 1978. 17(2): p. 321.
54. Tourtin, F., et al., Gallium phosphate thin solid films: structural and chemical determination of the oxygen surroundings by XANES and XPS. Thin Solid Films, 1998. 322(1-2): p. 85-92.
55. <The Chemistry and Binding Properties of alpo4.pdf>.
56. Ouillon, R., J.P. Pinan‐Lucarre, and P. Ranson, Anharmonicity of zone‐centre optical phonons: Raman spectra of the isomorphous α‐quartz, berlinite and gallium phosphate in the temperature range 8–300 K. Journal of Raman Spectroscopy, 2000. 31(7): p. 605-613.
57. Pinan‐Lucarre, J.P., R. Ouillon, and P. Ranson, Raman investigation of vibrational states in quartz‐type GaPO4 at various temperatures. Journal of Raman spectroscopy, 1998. 29(12): p. 1019-1026.
58. Souleiman, M., et al., Combined experimental and theoretical Raman scattering studies of α-quartz-type FePO 4 and GaPO 4 end members and Ga 1− x Fe x PO 4 solid solutions. RSC Advances, 2013. 3(44): p. 22078-22086.
59. Xia, H., et al., Growth and Raman scattering of aluminium gallium orthophosphate piezoelectric crystals. Progress in crystal growth and characterization of materials, 2000. 40(1-4): p. 253-261.
60. Kinyanjui, M., et al., Origin of valence and core excitations in LiFePO4 and FePO4. Journal of Physics: Condensed Matter, 2010. 22(27): p. 275501.
61. Kruse, J., et al., Phosphorus L2, 3-edge XANES: Overview of reference compounds. Journal of synchrotron radiation, 2009. 16(2): p. 247-259.
62. Wibowo, R.E., et al., Core-Level Spectroscopy with Hard and Soft X-Rays on Phosphorus-Containing Compounds for Energy Conversion and Storage. The Journal of Physical Chemistry C, 2023. 127(42): p. 20582-20593.
63. Yang, S., et al., Soft X-ray XANES studies of various phases related to LiFePO 4 based cathode materials. Energy & Environmental Science, 2012. 5(5): p. 7007-7016.
64. Liang, X., et al., Constructing a Z-scheme heterojunction photocatalyst of GaPO4/α-MoC/Ga2O3 without mingling type-II heterojunction for CO2 reduction to CO. ACS Applied Materials & Interfaces, 2021. 13(28): p. 33034-33044.
65. Anitha, N., et al., Influence of substrate temperature on the physical properties of SnS 2 thin films prepared using nebulized spray pyrolysis technique. Journal of Materials Science: Materials in Electronics, 2018. 29: p. 11529-11539.
66. Guo, C., et al., High-quality single-layer nanosheets of MS 2 (M= Mo, Nb, Ta, Ti) directly exfoliated from AMS 2 (A= Li, Na, K) crystals. Journal of Materials Chemistry C, 2017. 5(24): p. 5977-5983.
67. Jung, D.H., S.-i. Kim, and T. Kim, Characteristics of electrical metal contact to monolayer WSe2. Thin Solid Films, 2021. 719: p. 138508.
68. Romanov, R.I., et al., Thickness-dependent structural and electrical properties of WS2 nanosheets obtained via the ALD-grown WO3 sulfurization technique as a channel material for field-effect transistors. ACS omega, 2021. 6(50): p. 34429-34437.
69. Alyoruk, M.M., et al., Promising piezoelectric performance of single layer transition-metal dichalcogenides and dioxides. The Journal of Physical Chemistry C, 2015. 119(40): p. 23231-23237.
70. Brennan, C.J., et al., Out-of-plane electromechanical response of monolayer molybdenum disulfide measured by piezoresponse force microscopy. Nano letters, 2017. 17(9): p. 5464-5471.
71. Duerloo, K.-A.N., M.T. Ong, and E.J. Reed, Piezoelectricity in monolayers and bilayers of inorganic two-dimensional crystals. MRS Online Proceedings Library (OPL), 2013. 1556: p. mrss13-1556-w09-10.
72. Esfahani, E.N., et al., Piezoelectricity of atomically thin WSe2 via laterally excited scanning probe microscopy. Nano Energy, 2018. 52: p. 117-122.
73. Kim, S.K., et al., Directional dependent piezoelectric effect in CVD grown monolayer MoS2 for flexible piezoelectric nanogenerators. Nano Energy, 2016. 22: p. 483-489.
74. Lee, J.H., et al., Reliable piezoelectricity in bilayer WSe2 for piezoelectric nanogenerators. Advanced Materials, 2017. 29(29): p. 1606667.
75. Li, Y., et al., Tunable interlayer coupling and Schottky barrier in graphene and Janus MoSSe heterostructures by applying an external field. Physical Chemistry Chemical Physics, 2018. 20(37): p. 24109-24116.
76. Lu, A.-Y., et al., Janus monolayers of transition metal dichalcogenides. Nature nanotechnology, 2017. 12(8): p. 744-749.
77. Sharma, P., et al., A room-temperature ferroelectric semimetal. Science advances, 2019. 5(7): p. eaax5080.
78. Xue, F., et al., Multidirection piezoelectricity in mono-and multilayered hexagonal α-In2Se3. ACS nano, 2018. 12(5): p. 4976-4983.
79. Han, S.A., et al., Point‐defect‐passivated MoS2 nanosheet‐based high performance piezoelectric nanogenerator. Advanced Materials, 2018. 30(21): p. 1800342.
80. Khan, H., et al., Liquid metal-based synthesis of high performance monolayer SnS piezoelectric nanogenerators. Nature communications, 2020. 11(1): p. 3449.
81. Hutchings, G.S. and E.I. Altman, An atomically thin molecular aperture: two-dimensional gallium phosphate. Nanoscale Horizons, 2019. 4(3): p. 667-673.
82. Lemme, M.C., et al., Nanoelectromechanical sensors based on suspended 2D materials. Research, 2020.
83. Manzeli, S., et al., Piezoresistivity and strain-induced band gap tuning in atomically thin MoS2. Nano letters, 2015. 15(8): p. 5330-5335.
84. Wagner, S., 2D materials for piezoresistive strain gauges and membrane based nanoelectromechanical systems. 2018, Dissertation, RWTH Aachen University, 2018.