## 1 Introduction  
It is hard to overestimate the importance of high-performance photosensing devices and technologies for modern technical progress [1]. Typically, any photosensing device converts the incoming light to an electrical signal on the basis of photon absorption by a photosensitive material, with conversion of an optical into an electric signal realized. Important applications of such units as high-speed information transceivers and communication networks [2; 3], various types of remote sensing using the photosensitive matrices [4], and ultrafast sources of hot electrons in vacuum microelectronics devices [5; 6] should be mentioned. In particular, significant research efforts in recent years have been focused on the creation of novel photosensing material platforms, allowing for control of the bandgap width as well as an increase in the efficiency of light absorption and generation of hot electrons [2; 3; 7]. For example, the structures based on ultrathin layers of semiconductors such as transition metal dichalcogenides with a fundamental absorption in the near infrared (NIR) and visible regions possess very favorable properties for successful applications in photonics and optoelectronics [8]. The nanoscaled 2D layered structures based on the black phosphorus [9] opened up new possibilities for the bandgap control. This ensures the photosensing tuning within a broad wavelength range (from the visible to the infrared). It should be noted, however, that all the mentioned photosensors are based on the internal photoelectric effect. Photosensors based on the external photoelectric effect are required in the various areas of modern vacuum optoelectronics (for example, for rapid modulation of intense electron beams). In particular, the photocurrent in the vacuum under IR irradiation can be obtained using the structures based on the combination of low-dimensional semiconductor structures and plasmonic materials [10]. However, despite a high level of optical absorption and enhancement of excitation of plasmonic oscillations in these structures, there are problems related to significant electrical capacitance of the structure and, respectively, persistence of the charge transfer [10; 11]. These problems can adversely affect the performance of a photosensor. Photosensors with metal sensing elements are somewhat prominent in this series of devices. They have an undeniable advantage of a high rate of excitation of the electrons in the conduction band (hereinafter referred to as \"free electrons\") and their transport into the vacuum. However, owing to a relatively high work function, the photoeffect is observed only under a short-wavelength irradiation (in particular, in the ultraviolet region). Only using the surface plasmon resonance and/or deformation of the Schottky barrier at the metal-vacuum interface, the desired result can be achieved. In particular, this approach was successfully applied using nanosized gold plasmon structures [5; 6]. The periodicity of the arrays of planar emitters [5] and bulk emitters [6; 12] was reduced to 1 \(\upmu\)m; this value corresponds to the marginal possibilities of modern technologies based on the e-beam lithography. The gaps between the emitters were equal to 200 nm in the case of the planar structure, and 150 nm in the case of the bulk emitters. However, the necessity to provide a high-precision topology for such structures with the developed working surface makes the production of such devices very expensive and time-consuming. In addition, their functional properties become very sensitive to deviations in the technological parameters. On the contrary, the photosensor design discussed in this work is sufficiently less sensitive to the above mentioned technological problems. Its production is based on the standard planar technology [13; 14] and includes the photolithography, liquid etching, and magnetron sputtering deposition. In addition, the developed vacuum technology provides high performance and reproducibility of the devices.