Synthesis And Photoelectric Properties Modulation Of Two-Dimensional Hexagonal Boron Nitride

Solar light ranging from 200 to 280 nm is absorbed by earth’s atmosphere so it cannot reach the earth’s surface,deep ultraviolet photodetectors（DUVPDs）that detect this wavelength range of light are called solar-blind photodetectors.Due to this unique merit of a“black background”on the earth,there’re wide applications for solar-blind photodetectors,including missile warning,flame detection and spatial communications,etc.The research and development of solar-blind photodetectors is of great strategic significance to China’s territorial security and other fields.Solar-blind photodetectors mainly include silicon based photodetectors,whereas the Si channel possesses a narrow bandgap of 1.12 e V,costly high-pass optical filters are required to block out visible and infrared photons.As a new material that only has response to deep UV light,two-dimensional（2D）layered hexagonal boron nitride with a high UV to vidible light response rejection ratio,compensates the shortages of Si.Yet there are many challenges to synthesize large-scale and high-quality hBN with controllable thickness and modify its photoelectric properties.Here,we fabricate high-quality hBN with controllable thickness and size and doped hBN in a CVD system and reveal the modified mechanism by first-principles calculations.Untill now the systematic analysis of electronic structures and optical properties of doped hBN on the perspective of photoelectric properties modulation was still lacked.The photoelectric properties of hBN and doped hBN were studied and analyzed by first-principles calculations before the synthesis of hBN and its doping modulation to analyze the feasibility of doped regulating the photoelectric properties of hBN.The doped sites of nonmetallic H,C,O,F,Si,P,S and Cl atoms replacing B or N atoms in hBN were confirmed based on the formation energy.The regulation ability of these doped atoms was compared and analyzed from the aspects of electronic structures and optical properties,and then figured out the suitable doped atoms.All these atoms doping can enhance the conductivity and light absorption coefficient at 260 nm-280 nm of hBN.The S or C atoms are ideal doped atoms due to its excellent photoelectric properties modification abilities and being easy to obtain and operate.These prospective theoretical calculations will provide a theoretical basis and guidance for the synthesis of hBN and its photoelectric properties engenering.Since the thickness of hBN films synthesized on Fe,Ni and their alloys is normally inhomogeneous,we created a new method to sybthesize uniform multilayer hBN films with a controllable thickness on Au foils.The growth mechanism of clustering nucleation followed by transverse epitaxial growth of multilayer hBN films on Au foils was revealed by systematically studied its time evolution of growth process and growth factors of substrate,reaction temperature,carrier gas ratio,reactant concentration,etc.Due to its high crystallinity,cleanliness and uniformity,the responsivity rejection ratio of 220 nm to 290 nm(R_{220 nm}/R_290nm>10⁴)of the constructed solar-blind hBN photodetectors is superior to that of most previous solar-blind photodetectors based on traditional semiconductors（≈10³）,and the mechanism of enhancing photoelectric properties of hBN devices by thickness is revealed from the aspect of electronic structures.These studies not only provide a new route for the controllable fabrication of multilayer hBN films with uniform thickness,but also promote the development of solar-blind photodetectors and other electronic devices.To extend the light response range of solar-blind hBN photodetectors,a～2×2cm² S-hBN single-crystal film was firstly synthesized on S-resistant liquid Au substrates.We liquefied the Au substrate to increase its flatness and eliminate grain boundaries,adjusted the distance of the reactant and substrates to obtain circular S-hBN single crystals,controlled the ratio of gas to obtain precisely aligned S-hBN single crystals,increased reaction temperature to inprove the crystallinity of S-hBN single crystals.The growth mechanism of dispersed nucleation,competitive growth,precise alignment and mutual merging were revealed by studied its time evolution of growth process and crystal orientation.Combined with theoretical calculations,the mechanism of regulating the photoelectric performance of hBN by S doping is studied.After S doping,the hybridization of B 2p with S 3p orbitals forming a new conduction band edge narrows the bandgap that improves its light absorption and electrical conductivity.The cut-off light response is extended from 260 nm to 280nm,and the photocurrent and responsivity of S-hBN are～50 times higher than that of pristine hBN under 280 nm irradiation.This study provides a new method for the preparation of large-area doped hBN and other doped two-dimensional materials.Strain coordination with C doping was used to engineer the photoelectric performance of hBN due to the limitation of S doping modulation.By regulating the ratio of gas,increasing reaction temperature and the distance of the reactant and substrates,large-size triangular C-hBN single crystals were obtained.The C doping concentration in hBN single crystals is effectively regulated by controlling the flow rate of CH₄.The quantitative relationship（Eg=-0.68ln N+6.78）of bandgap and C doping concentration is also revealed.The effect of strain on photoelectric properties of C-hBN is further studied.Under 0.62%tensile strain,the responsivity(27.2 m AW^-1)is much higher than that of hBN and doped hBN photodetectors(9.2m AW^-1)reported previously.The photoelectric performance enhanced mechanism of C-hBN by tensile strain was revealed by experiments and theoretical calculations.It shows that strain enhancing the performance of C doped hBN photodetectors is an effective strategy,which provides a new idea for designing high performance solar-blind photodetectors.