Nowadays，semiconductors are widely used in energy storage，environmental sensing，neuromorphic systems，etc^［1-3］. Among these applications，photodetectors with the ability to convert optical signal into electrical signal，play an important role in sensing network system including imaging，night vision，optical communication and security inspection^［4-7］. At present，the most widely used is silicon-based commercial photodetector，which exhibits fast response speed，high responsivity and large signal-to-noise ratio. However，its high intrinsic band gap（1.12 eV）limits the detection range to below 1100 nm. To fulfill the needs of infrared imaging and other fields，it is urgent to develop high performance broadband infrared photodetectors. To date，most of the commercial infrared photodetectors are based on traditional materials such as InGaAs，InSb，HgCdTe^［8-11］. However，the expensive material growth procedure and the dependence on refrigeration equipment to reduce dark current restrict their further application. Therefore，it is necessary to develop an infrared detector that is easy to use，inexpensive，and of good performance.

As a typical P-type semiconductor，Se（Selenium）exhibits many excellent optoelectronic properties，such as fast response speed and large photoconductivity. The first photoconductor based on Se was reported by Smith in 1873^［12］. After that，virous Se-based photodetectors have been designed. Karim et al. used amorphous Se to convert and detect the X-rays^［13］. Pan et al. systematically studied the effects of hole-blocking dielectric layer on the microstructure and visible photoconductive properties of polycrystalline Se thin films^［14］. In addition to thin films，various nanostructures have also been synthesized. Due to the chain-like molecular structure，Se spontaneously delivers 1D morphologies. Fang et al. fabricated a plasmonic enhanced t-Se microtube photodetector^［15］. Wang et al. constructed a Se nanobelt-based transistor，where the hole mobility and concentration were estimated to 0.63 cm² （V s）^-1 and 9.35×10¹⁶ cm^-3 respectively^［16］. These studies have promoted the application of Se in photodetection. However，the intrinsic wide bandgap of Se（1.77 eV）limits its application in the infrared range. To further improve the detection range of the device，a common approach is to construct heterojunctions with other infrared materials^{［17，18］}. In addition，using defect absorption to improve the detection range is also an effective method. For example，blocked impurity band（BIB）infrared detectors employ a layer of material doped with specific impurities to create defect states within the bandgap，which can absorb specific wavelengths of infrared radiation. The absorbed radiation generates electron-hole pairs，which are collected by an electric field and converted into an electrical signal，enabling the detection of various wavelengths of infrared radiation^［19］. However，BIB detectors rely on complex doping processes. In contrast，employing defects that spontaneously generated during material growth to achieve infrared absorption is a simpler approach. In our previous work，a broadband photodetection up to 1550 nm has been obtained based on Se self-supporting polycrystalline film，which demonstrates the potential of selenium in infrared detection^［20］. However，since the thin film has an amorphous structure，the recombination of carriers at the grain boundary significantly reduces the responsivity of the device.

In this work，we employed a low-temperature and catalyst-free method to prepare one-dimensional Se microrods with a single crystal structure，which is advantageous for efficient carrier transport. After constructing electrodes at each end of the microrod with conductive silver paste，a MSM（metal-semiconductor-metal）type photodetector has been fabricated. During the photodetection test，the device exhibits broadband detection spectrum（450 ~ 1550 nm），fast response speed（T_r = 41 ms，T_d = 46 ms），large responsivity（18.32 mA/ W）and detectivity（1.65×10⁸ Jones）. Besides，it can also exhibit self-powered photodetection capability under zero bias due to the different Schottky barrier height between the two ends of the microrod and the electrode. These results fully demonstrate the application of Se in broadband photodetection.

Materials growth and characterization：Se microrods were prepared using the physical vapor deposition（PVD）method. Fig. 1（a）illustrates the schematic diagram of the synthesis process. Firstly，high-purity Se powder was placed in the quartz boat. Subsequently，the quartz boat was pushed to the heating center of the horizontal tube furnace，and a piece of clean glass was placed down steam. To remove air，the tube was pumped and flushed with Argon and repeated three times. During the growth，the tube furnace was heated to 250℃ in 50 min and held for 2 hours with 100 sccm Argon flow. Following the growth period，the furnace was allowed to cool naturally to room temperature. As a result，bright black Se products were obtained on the glass substrates.

The crystal structure was characterized by the powder X-ray diffraction（Empyrean，PANalytical Company），and the scanning range 2θ is 10°~90°. The morphology was obtained by the Scanning Electron Microscope（MIRA3，Czech Tesken Company）. The optical absorption spectrum was measured by UV-Vis-NIR spectroscopy（SolidSpec-3700，SHIMADZU Company）. The Raman spectrum was collected on a Raman spectrometer with an excitation wavelength of 532 nm（LabRAM HR800，Horiba JobinYvon Company）.

Device fabrication：With the help of the surface tension of water，a single Se microrod with smooth surface and uniform diameter can be attached to the metal needle through a drop of water and transferred onto the glass substrate. After that，the conductive silver paste was uniformly coated on both ends of the microrod. Finally，the device was annealed at 110℃ in high-purity Argon atmosphere to improve interface contact quality. The schematic diagram and optical image of the device are shown in Fig. 1（b）and Fig. 1（c）.

Device Performance Characterization：The photoresponse was measured by the Semiconductor analyzer（Keithley 4200A-SCS）system and monochromatic light source. The light source is a series of single-mode fiber-pigtailed lasers of different wavelengths，which illumination intensity and light on-off frequency can be controlled by the current and temperature controller（ITC4001）.

Fig. 2（a）shows the scanning electron microscope（SEM）image of a Se microrod on conductive carbon tape. The diameter of the microrod is about 35 μm and the surface is smooth，indicating that the Se microrods are suitable for making photodetectors by hand. To verify the crystal structure of the product，X-ray diffraction was collected，and the corresponding pattern is shown in Fig. 2（b）. All diffraction peaks can correspond to the standard pattern of Se（PDF#06-0362），which indicates the high purity of our products. From the pattern results，the synthesized microrods exhibit a triangular structure，and the lattice constants are a=b=4.366Ao，c=4.954Ao.

The optical absorption spectrum of as prepared Se microrods within 350 - 1800 nm is shown in Fig. 2（c）. The light absorption of Se products was higher in the wavelength range of 350 ~ 680 nm but decreased significantly when the wavelength larger than 680 nm. From the illustration in Fig. 2（c），the band gap of Se microrods is about 1.71e V，which is consistent with the bulk Se. From the Raman spectrum in Fig. 2（d），their two strong Raman peak located at 144 cm^-1 and 237 cm^-1. The Raman resonance peak at 237 cm^-1 is the A₁ mode，which is the characteristic stretching mode of the chain-like structure and only exists in triangular selenium. The other peak located at 144 cm^-1 is the E₁ mode，corresponding to the transverse optical phonon mode. This Raman result is consistent with the XRD pattern in Fig. 2（b），indicating the synthesized Se microrods belong to triangular selenium.

To study the photoelectric properties of the selenium microrod. A series of photoelectric tests were carried out. Fig. 3（a）shows the current voltage（I-V）curves of the Se microrod photodetector in dark and under 450 nm light illumination. It is clear that the current increases significantly after illumination. As the light intensity is increased，the current also increases synchronously. However，when the intensity is increased to 2.958 mW/mm²，the photocurrent tends to saturate. In the case of the Se microrod photodetector，it works based on the principle of photoconductivity. When it is illuminated by light，the absorbed photons create electron-hole pairs. The generated electrons and holes can then be separated by the built-in electric field within the material and contribute to the electrical current that flows through the device. As the light intensity increases，more photons are absorbed，and more electron-hole pairs are generated，leading to an increase in the electrical current. However，when the light intensity reaches a certain level，the number of electron-hole pairs becomes saturated，and the photocurrent no longer increases. Besides，when rising the bias voltage，the photocurrent also increases，which shows a synergistic effect between light and potential. Fig. 3（b）shows the transient response behavior of the photodetector under 450 nm light intensity of 3.845 mW/mm² at a 3 V bias voltage，which shows the device exhibits a fast and stable switching characteristic. The response speed is also an important performance parameter. By intercepting part of the I-T curve，the rise and decay time in Fig. 3（c）are calculated to be 41 and 46 ms. In addition to the above parameters，the responsivity（R）and detectivity（D*）are also two important indicators to evaluate the photo detecting performance. Where R is the efficiency of the photodetector in converting the optical signal into electrical signal，and D* is the detect potential of weak light signals. These two important parameters can be derived using the following equations ^［21-23］.

where Iph is the photocurrent，Id is the dark current，P is the optical power density of the incident light source，S is the effective area of the detector irradiated by light，and e is the basic charge. To quantify the two key parameters，the responsivity and detectivity curves of the detector under 3 V and -3 V bias voltages and different light intensities are plotted in Fig. 3（d）. The results show that the responsivity and detectivity of the selenium microrod photodetector at the same bias voltage decrease with the increment of light intensity. This is because when the light intensity is too high，the number of photons absorbed by the photodetector exceeds the number of available carriers，leading to carrier saturation. This causes the excess photons to be wasted as heat instead of contributing to the photocurrent，resulting in a decrease in responsivity and detectivity. Under 3 V bias voltage，the maximum Rand D*are calculated to be 18.32 mA/W and 1.65×10⁸ Jones. At the reverse bias voltage of -3 V，the calculated results are 21.33 mA/W and 1.91×10⁸ Jones. The above differences can be explained by the I-V curves in Fig. 3（a）. The asymmetric curves show the presence of the Schottky barrier between the material and the electrode. As the work function of the pure Ag metal is about 4.6 eV，it tends to form ohmic contact with Se. But for the silver paste，it is a complex mixture and difficult to maintain a fixed work function. Kinds of literatures have reported that silver paste can form both ohmic contact and Schottky contact with the same semiconductor material^{［24，25］}. For our device，the difference in barrier height and contact area leads to different built-in potential at both ends of the electrode，which leads to the above different results.

To investigate the broadband detection potential，we test the photocurrent under laser illumination with different wavelengths from 785 to 1550 nm. It can be seen from Fig. 4（a）that the device has good detection performance for all the five selected light sources，and these results promote the detection potential of Se to the infrared range. Fig. 4（b）shows the Rand D*of the device under each light illumination. Compared with the test results under 450 nm illumination，the above results are much smaller. Since the above wavelengths exceed the intrinsic absorption cutoff wavelength of selenium，the above photogenerated carriers can be attributed to defect absorption. For the Se nanostructures synthesized by thermal evaporation，it usually forms an amorphous layer on the surface. The amorphous layer is easily to be terminated by H-，O-，OH-，thus forming surface defects^{［12，16，20］}. As the defect energy level is only about 0.4 to 1.0 eV，the absorption range can be greatly improved. However，the low absorption efficiency of the defects leads to lower performance indicators for photodetection.

To further expand the application range of devices，we tested the self-powered detection capability of the device. Although the device is a photoconductor，the different barrier height enables it to have the potential of self-powered detection. Fig. 4（c）exhibits the I-T curve of the device under 450 nm illumination without bias voltage，a fast and stable switching characteristic can also be obtained. Further response speed analysis in Fig. 4（d）shows that the rise time and decay time of the device is 8 ms and 12 ms，respectively.

As shown in table 1，we present a comparison of the optoelectronic performance indicators of our device with those of other photodetectors based on selenium materials. Although our device's performance metrics are not as impressive as those reported in the literature，it does exhibit a wide spectral response. Interestingly，when compared to other devices with a similar spectral response，our device's metrics appear to have certain advantages. Further research could be conducted to optimize the performance of our device and to investigate the underlying mechanisms that give rise to its unique characteristics.

In conclusion，we have fabricated a MSM photodetector based on Se microrods with a simple materials growth method and low-cost device fabrication process. Compared with traditional Se based photodetectors，the device has a broadband photodetection from visible to infrared range. In addition，due to the difference of Schottky barrier at both ends of the electrode，the device can also perform self-powered photodetection without bias. The research results of this paper not only improve the application of Se in infrared detection field，but also expand the application flexibility of device.