Surfing Silicon Nanofacets for Cold Cathode Electron Emission Sites

Abstract

Point sources exhibit low threshold electron emission due to local field enhancement at the tip. In the case of silicon, however, the realization of tip emitters has been hampered by unwanted oxidation, limiting the number of emission sites and the overall current. In contrast to this, here, we report the fascinating low threshold (∼0.67 V μm) cold cathode electron emission from silicon nanofacets (Si-NFs). The ensembles of nanofacets fabricated at different time scales, under low energy ion impacts, yield tunable field emission with a Fowler-Nordheim tunneling field in the range of 0.67-4.75 V μm. The local probe surface microscopy-based tunneling current mapping in conjunction with Kelvin probe force microscopy measurements revealed that the valleys and a part of the sidewalls of the nanofacets contribute more to the field emission process. The observed lowest turn-on field is attributed to the absence of native oxide on the sidewalls of the smallest facets as well as their lowest work function. In addition, first-principle density functional theory-based simulation revealed a crystal orientation-dependent work function of Si, which corroborates well with our experimental observations. The present study demonstrates a novel way to address the origin of the cold cathode electron emission sites from Si-NFs fabricated at room temperature. In principle, the present methodology can be extended to probe the cold cathode electron emission sites from any nanostructured material.

Growth of Wafer-Scale ReS2 with “Tunable” Geometry toward Electron Field-Emission Application

“…The transition from the thin film to vertical layers because of increasing T s is presented schematically in Figure a. KPFM is a nondestructive scanning probe technique that works based on an electrostatic interaction, which is generated due to a difference in the work functions between the metal-coated AFM tip and sample, as is depicted in Figure b. ,, This electrostatic force can be minimized by applying an opposite external dc voltage. This externally applied dc voltage is equivalent to the contact potential difference ( V CPD ) and is mathematically defined as: V CPD = (ϕ s – ϕ m )/ e , where ϕ s and ϕ m are the work functions of sample and metal-coated AFM tip, respectively (see Figure c). , Figure d and e shows the surface topography and corresponding ϕ s map for the RT-grown ReS 2 thin film, respectively.…”

“…Note that, in FE electrons tunnel from the cathode (e.g., sample) to the metallic anode through vacuum as per quantum phenomenon upon applying a bias, as is presented in Figure a. , Mathematically, FE is explained based on the Fowler–Nordheim (F–N) equation as: , where J is the emitted current density, A = 1.56 × 10 –10 A V –2 eV and B = 6.83 × 10 3 V eV –3/2 μm –1 , β is the geometric field-enhancement factor, and E is the applied field. ,,, This equation indicates that the J strongly depends on ϕ s , and to have low operative field and significant current density, a low ϕ s is desired. A schematic diagram of the FE setup is depicted in Figure b.…”

“…Both these samples, for example, T 200 and T 400 , have a minimum work function of 4.3 eV at the edges and thus, it is expected that both must show similar FE properties. However, measured different E th values indicate that other factors are also responsible for such tunable FE properties, including the low work function . In fact, it has been demonstrated that the geometrical factors such as shape and size also play important role in determining FE properties, which is defined by geometric field-enhancement.…”

“…A geometric field-enhancement, β, is usually defined as the resultant local field ( E loc ) applied to a nanostructure because of its geometry and is defined as: E loc = β E . In general, β is 1 for flat surfaces and greater than 1 for other geometries. ,, Using the F–N formalism, mathematically β is estimated from the slope ( m ) of ln­( J / E 2 ) versus (1/ E ) as: β = [−6.8 × 10 3 ϕ s 3/2 ]/ m . The ln­( J / E 2 ) versus (1/ E ) plots for all three samples are presented in Figure d.…”

“…FE is a quantum phenomenon in which the electron tunnel from the material to tip via vacuum potential barrier because of a strong applied electric field, typically in the range of 10 6 to 10 7 V cm –1 . − FE is the vital building block for diverse technological applications, such as flat displays, vacuum microelectronics, X-ray sources, high-energy accelerators, and electron guns. − ,, Theoretical prediction and experimental demonstration confirm that the FE properties can be improved/enhanced by geometrical modulation . For example, three-dimensional (3D) nanostructures manifest strong enhancement in performance through their tunable features of shape and size-different from the conventional planar film structures. ,, On the other hand, vertically grown 2D materials have sharp protruding edges and thus, most of the available studies FE performance is explained on this basis. − , Particularly, it could become more prominent for vertically grown ReS 2 layers because its individual layer can work as an electron emitter because of weakly interacting nature between the neighboring layers. , However, it remains a great technical challenge to deposit 2D ReS 2 with perpendicular edges to the substrate over a large area with high reproducibility …”