Highly Flexible and Robust N-Doped Sic Nanoneedle Field Emitters

Supplementary Information

Highly Flexible and Robust N-doped SiC Nanoneedle Field Emitters

Shanliang Chen1,2, Pengzhan Ying2, Lin Wang1, Guodong Wei1, Fengmei Gao1, Jinju Zheng1, Minhui Shang1, Zuobao Yang1, Weiyou Yang1,* and Tom Wu3,*

1 Institute of Materials, Ningbo University of Technology, Ningbo City, 315016, P.R. China.

2 School of Material Science and Engineering, China University of Mining and Technology, Xuzhou City, 221116, P.R. China.

3 Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia.

Corresponding authors. E-mails: (W. Yang) and (T. Wu)

Figure S1. Molecular structure of the precursors polysilazane (PSN).

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Figure S2. Typical SEM images of the carbon fiber substrate before (a) and after (b) growing SiC nanostructures of sample S1. (c) The SAED pattern recorded from the SiC nanoneedle. (d) Element mapping of N dopants within the body of SiC nanoneedle.

Figure S3. (a) A typical SEM image of the obtained quasialigned SiC nanowires arrays of sample S2. (b) A typical TEM image showing the tip of an individual SiC nanowire. (c-d) Corresponding SAED pattern and HRTEM image recorded from the marked area of A in (b). (e) A representative element mapping of N dopants within the SiC nanowire.

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Figure S4. Typical SEM images of the obtained sample S3 under different magnifications.

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Figure S5. A representative EDS spectrum recorded from the catalytic particle of sample S2 (marked area of B in Figure S3b).

Proposed Mechanism for Doping Controlled Growth of SiC Nanowires

The growth of the SiC nanowires could be briefly described by the step-by-step schematics shown in Figure S4(a): i) PSN polymeric precursors are converted into SiCN(O) amorphous ceramics when heated up to 1000 °C1 and the vapor phases such as SiO and CO are released as the temperature continues to rise (shown as step I);2 ii) With the further increase of the temperatures, the vapor phases would react with the molten catalyst, leading to the formation of Co catalyst alloy droplets on the substrate surface (shown as step II); iii) During the decrease of the pyrolysis temperatures, the continuous dissolution and precipitation of vapor phases within the liquid catalytic droplets would initiate the nucleation and growth of SiC nanowires (shown as step III-V). Meanwhile, the N atoms coming from the gas mixtures would undergo the similar dissolution and precipitation process, which leads to the incorporation of N into the crystal lattice via the formations of substitutional solid solutions and/or interstitial solid ones (shown as Figure S4(b)).3 The correlation between the N doping levels of ~4.39, 5.78 and 7.32 at% and the morphologies of the samples S1, S2 and S3 implies that a higher concentration of N2 within the N2/Ar gas mixture could favor a bigger catalyst droplet and higher N doping level in SiC nanostructures. These could be attributed to two main reasons. One is the different physical properties between N2 and Ar (e.g., the respective thermal conductivities of N2 and Ar at room temperature is 0.02475 and 0.01795 W (m×k)-1). For the growth of SiC nanostructures based on the VLS process, the catalyst droplets cannot be considered as an adiabatic or isothermal system, because of the radiative losses and conductive heat transfer between the droplets and surrounding vapors.4 The higher N2 content with a higher thermal conductivity around the catalyst droplets would facilitate the heat transfer (shown as Figure S4(c)) and makes the droplet temperature drops faster, which allows the droplet temperature closer to that of the furnace, thus weakening the thermal evaporation and resulting in the grown SiC nanowires with a larger catalyst droplet and body diameter. The other fact is that a higher N2 bias pressure could makes more N atoms dissolved into the liquid catalyst droplets, and in turn, resulting in a higher N doping levels and a bigger size of the liquid catalyst droplets, which leads to the growth of the SiC nanostructure with bigger catalytic droplets and nanowires diameters (Figure 1, 3 and Figure S2 and S3). These results clearly suggest that the gas mixture compositions with various ratios of N2:Ar play a profound role on the growth of flexible quasialigned SiC nanoarrays, in which the morphologies and N-doping levels can be tailored by adjusting the gas mixture compositions.

Figure S6. Schematic illustration for doping controlled VLS growth of SiC nanostructures.

Figure S7. Schematic diagram of the experimental setup used for the measurement of field emission properties of SiC field emitters.

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Figure S8. (a-c) The typical SEM images of N-doped SiC nanoneedles after FE operation of sample S1, S2-1 and S3-1.

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Table S1. Turn-on and threshold fieldsa of typical nanostructured flexible emitters.

Emitters / Turn-on fields (V/μm) / Threshold fields (V/μm) / Ref.
Typical flexible emitters based on carbon fabric substrates / n-type SiC nanoneedles flexible emitters / 1.11-1.38 / 1.55-1.79 / this work
Oriented CuO nanoknife arrays / 0.9 / -- / 5
SnO2 nanowires arrays / 3.3 / 4.5 / 6
N-doped 3C-SiC nanoneedles / ~1.1 / -- / 7
Patterned carbon nanotube arrays / 0.4 / 0.84 / 8
Tapered SiC Nanowires / ~1.2 / -- / 9
Carbon nanoparticles / 1.33 / 2.8 (10 mA/ cm2) / 10
Highly wrinkled graphene / 1.18 / 1.43 (0.2 mA/ cm2) / 11
Tungsten oxide nanowires / -- / 4.3 (10 mA/ cm2) / 12
Typical SiC nanostructured field emitters on rigid substrates / B-doped 3C-SiC nanowires / 1.35 / 1.70 / 13
N-doped nanoporous SiC / 4.4-9.6 / -- / 14
Well-aligned SiC nanowires arrays / 1.3 / 2.2 / 15
Al2O3-decorated tubular SiC / 2.4 / 5.37 (10 mA/ cm2) / 16
Tubular β-SiC / 5 / 10 (10 mA/ cm2) / 17
Aligned SiC porous nanowires / 2.3-2.9 / -- / 18
β-SiC nanowires / -- / 4 / 19
β-SiC nanoarchitectures / 12 / -- / 20
Bamboo-like β-SiC nanowires / 10.1 / -- / 21
SiC nanowires/nanorods / 3.33 / 5.77 (10 mA/ cm2) / 22
Nonaligned SiC nanowires / 3.1-3.5 / -- / 23
Core-shell SiC-SiO2 nanowires / 3.3-4.5 / -- / 24
Typical inorganic semiconductor nanostructured emitters / Vertically aligned LaB6 nanowires arrays / 1.82 / 2.48 / 25
Conical shape GaN nanorods / 3.35 / -- / 26
SnO2 nanowires / 3.5 / 4.63 / 27
WO3 nanowires / 1.8 / 3.3 / 28
Ultrathin BN nanosheets / 1.9 / 4.65 (10 mA/ cm2) / 29
Single-crystalline CdS nanobelts / 3.7 / 9.3 / 30
Ordered CdS nanostructure arrays / 12.2 / 15.7 (0.1 mA/ cm2) / 31
Single-crystalline PrB6 nanorods / 0.95-2.80 / 3.55-6.99 / 32
Ultrafine ZnS nanobelts / 3.47 / -- / 33
ZnS nanobelts quasi-arrays / 3.55 / -- / 34
Si nanowires / -- / 0.7 / 35
Aligned untralong ZnO nanobelts / 1.3 / 2.9 / 36
Si-doped AlN nanoneedle array / 1.8 / 4.6 (10 mA/ cm2) / 37
TaSi2 nanowires / 4-4.5 / 6 (10 mA/ cm2) / 38
Co5Ge7 nanowire and nanobelts arrays / 1.6 / -- / 39

a The turn-on and threshold fields required to generate an emission current density of 10 μA/cm2 and 1 mA/cm2, respectively. If other values are used, it will be mentioned separately.

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