Grassy Silica Nanoribbons and Strong Blue Luminescence

Silicon dioxide (SiO2) is one of the key materials in many modern technological applications such as in metal oxide semiconductor transistors, photovoltaic solar cells, pollution removal, and biomedicine. We report the accidental discovery of free-standing grassy silica nanoribbons directly grown on SiO2/Si platform which is commonly used for field-effect transistors fabrication without other precursor. We investigate the formation mechanism of this novel silica nanostructure that has not been previously documented. The silica nanoribbons are flexible and can be manipulated by electron-beam. The silica nanoribbons exhibit strong blue emission at about 467 nm, together with UV and red emissions as investigated by cathodoluminescence technique. The origins of the luminescence are attributed to various defects in the silica nanoribbons; and the intensity change of the blue emission and green emission at about 550 nm is discussed in the frame of the defect density. Our study may lead to rational design of the new silica-based materials for a wide range of applications.


Results
shows a representative Helium ion microscopy (HIM) 18 image of the silica nanoribbons grown on SiO 2 /Si substrate. The gatherings of nanoribbons with a length of tens of micrometers and varying widths resemble bunches of chlorophytum comosum planting into the ground. We further transferred several nanoribbons from the SiO 2 /Si substrate onto transmission electron microscope (TEM) Cu meshes to investigate the fine microstructures of the nanoribbons. As displayed in Fig. 1b, the silica nanoribbons are electron-transparent. No ordered structure is observed in the high-resolution TEM image ( Fig. 1c and Figure S1 in Supp. Info.), suggesting an amorphous nature of the silica nanoribbons. We had also annealed the nanoribbons for 2 h at 1000. However, amorphous to crystalline structure transition did not occur. The amorphous feature of the nanoribbons is confirmed by the X-ray diffraction (XRD) pattern of the samples (see Figure S2, Supp. Info.). This phenomenon could be attributed to the multilayered structure of the silica nanoribbons, since it has been reported that growing additional layers on top of the crystalline silica monolayer would finally result in amorphous silica films 19 . The thickness of individual nanoribbons based on atomic force microscopy (AFM) measurements ranges from 5.4 nm (Fig. 1e) to approximately 50 nm, becoming thinner along the growth direction of the grassy nanoribbons (Fig. 1d,e).
We characterized the chemical composition and Si oxidation states of the silica nanoribbons by employing local scanning Auger electron spectroscopy (AES) technique 16,[20][21][22] . The Si LVV AES peak position and shape are extremely sensitive to the Si oxidation state 21 . The main peak for elemental Si is centered around 90 eV while that for SiO 2 is located at 76 eV 21,23 . Figure 2b depicts the AES spectra acquired at different sample locations in Fig. 2a. At the substrate region, i.e., #1 position, the Auger electron signals of both the elemental Si and the Si oxidation states for SiO 2 were detected. By contrast, only the latter was detected on the nanoribbons, i.e., #2 and #3 positions. Figure 2c,e show the Auger electron maps of the Si LVV (Fig. 2d) and O KLL (Fig. 2e), elucidating the chemical distribution of the elements in the silica nanoribbons. Besides, the X-ray photoelectron spectroscopy (XPS) results corroborate the formation of silica nanoribbons ( Figure S3, Supp. Info.). Raman spectrum ( Figure S4, Supp. Info.) and Fourier transform infrared spectroscopy (FTIR) ( Figure S5, Supp. Info.) techniques were also used to analysized the silica nanoribbons in ambient air.

Discussion
We found the formation of silica nanoribbons when heating very thin Cr films deposited on SiO 2 /Si substrates under S atmosphere. However, without either Cr or S, no silica nanoribbon could be obtained. It is a big surprise, to some degree, because a thin Cr layer is normally an adhesive layer of the source/drain electrodes for field-effect transistors on SiO 2 /Si platform 24 . In the literature, the production of monolayer or bilayer 2D silica depended only on the metals and no S was present in their reaction systems [11][12][13][14][25][26][27][28] . Therefore, we believed that Cr and S acted as the dual co-catalysts for the growth of silica nanoribbons. Furthermore, on the basis of the morphological observation of the different phases of the growth, the growth mechanism of silica nanoribbons is most likely to be as follows (see Fig. 3 and Figure S6 in Suppl. Info.). Cracks were firstly generated in the SiO 2 layer on Si with the assistance of Cr and S at high temperature (Fig. 3a); melting of the SiO 2 occurred at the wall of the SiO 2 cracks; and nanoribbons grew in-situ at the crack sites. It is also sound from the observations that the Si and O sources for the formation of silica nanoribbons were originated from the melted SiO 2 . The formed silica nanoribbons exhibit various features. Some nanoribbons like a braid (Fig. 3b,c), and some have wedge-shaped edges (Fig. 3d). It is also observed that long silica nanowires with size of about 20 nm ( Figure S6e, Supp. Info.) were produced together with the nanoribbons (see Figs 2c-e and 3). In addition, short silica rods were also produced, lying on the substrate (see Figs 2c-e and 3). The generation of these silica nanostructures on the SiO 2 substrate is rather interesting and the grassy appearance of the silica nanostructures is quite astonishing. The silica nanoribbons are bendable. Moreover, the nanoribbons are sensitive to the exposure of electron beam. As demonstrated in the sequentially acquired AES mappings in Fig. 2c-e, the nanoribbons highlighted in the oval were movable under the electron beam irradiation, which indicates the possibility to manipulate the nanoribbons by electron beam. Also, excessive exposure of electron beam could damage the nanoribbons as observed in Fig. 3b,c. Our grassy silica nanoribbons show distinct morphological characteristics from previous reported silica nanostructures such as nanowires 29 and twisted nanobelts and nanosprings 30 synthesized by a thermal evaporation method at 1300 °C, porous nanostructures synthesized by complicated solution chemical reactions 31 , and silica nanotubes synthesized by a template-directed method 32 .
Defects in SiO 2 exert significant influences on its properties such as dielectric performance and luminescence. The defect structure of SiO 2 is extremely sensitive to ionizing radiation 33,34 . Several kinds of defects in amorphous SiO 2 are optically active and can be studied by luminescence spectroscopy. We investigated the defect-induced luminescence properties of the grassy silica nanoribbons by CL spectroscopy, which is a frequently used technique for high spatial resolution and high-sensitivity detection of defect centers in materials. Figure 4 displays the CL characterization results. All the CL spectra (#2-#9 in Fig. 4a,b) acquired from the different sample locations of nanoribbons possess luminescence features at around 285 nm (4.35 eV, UV band), 467 nm (2.60 eV, blue band), 550 nm (2.25 eV, green band), and 645 nm (1.92 eV, red band) 33 . No blue emission can be detected from the substrate (#1). These luminescent bands are common in silica and the specific luminescence centers related to them have been well documented [35][36][37][38] , originating from local atomic rearrangement that deviates from the SiO 4 tetrahedra expected for a perfect silica matrix. The red emission band ascribed to the nonbridging oxygen hole centers (NBOHCs) 39,40 (see Fig. 4c). The UV emission band can be originated from particular kinds of oxygen vacancy centers (OVCs) such as the discoordinated Si or neutral oxygen vacancy (Fig. 4c) 35,37 .
The most striking change in the present set of CL spectra is the luminescent intensity of the blue and green emissions (Fig. 4a,b). At the substrate location (#1), no blue emission appears, in line with previous study on   unirradiated amorphous SiO 2 41 . However, at the apex (#2 as well as #3) of the upright, free-standing silica nanoribbon (Figs 3e and 4a), very strong blue emission is observed. This comparison suggests that the blue emission is most likely to be orginated from surface features of the high surface area silica nanoribbons rather than the bulk features of silica. Thus, on the basis of previous study on the blue emission of silica nanostructures [42][43][44][45] , the observed blue emission here could be associated with a defect pair consisting of dioxasilirane (= Si (O2)) and a silylene (= Si:). As for the green emission, this may be associated with oxygen-related defects such as silanone (= Si= O) or dioxasilyrane 40 .
The intensity change of the blue and green emissions is clearly illustrated by the luminescent spectra acquired at the #4, #5 and #6 locations. These phenomena can be explained based on the origins of the luminescence as discussed above. On the one hand, we confirmed the remarkable defective nature of the silica (SiO x ) nanoribbons by TEM-based energy dispersive spectroscopy (EDS) measurements. An atomic ratio ranging from 1.54 to 2.09 between O and Si, typically smaller than 2.0, was obtained for the SiO x nanoribbons ( Figure S7, Supp. Info.). This substoichiometric characteristic suggests the existence of oxygen vacancies in the silica nanoribbons, leading to the strong blue emission. On the other hand, as shown in the AES spectra (Fig. 2b), elemental Si signal was detected at the substrate, which indicates the existence of Si aggregates at the substrate surface resulted from the silica production as addressed above. Therefore, strong green luminescence also appears at the substrate. The intensity mappings of the blue and green emissions are displayed in Fig. 4d,e, respectively. The map of blue emission matches well with the silica nanoribbons (Fig. 4a). The reason resulting in the intensity change of blue and green bands might be the variation of the oxygen content in the silica nanoribbons. Although a quantitative correlation between the oxygen content and the emission intensity is difficult to be concluded at this stage, the lower oxygen content, responsible to the blue emission, could suggest a higher component of silicon aggregates, responsible to the green emission, in the silica nanoribbons. In fact, the OVCs could be described as Si-Si links in the form of dimers, timers or hexamers 42,46 . In this perspective, the very weak blue emission at #4 is likely due to the higher content of oxygen than those at #5 and #6 locations.
We investigated the CL properties of a gathering of silica nanoribbons as a function of irradiation time (Fig. 5). The CL emission was excited by continuous irradiation over the observation field with an electron beam of energy 5.0 keV and current 0.1 nA. The very weak green emission of the first collected CL spectrum might be due to the relatively small substrate area because irradiated area is almost fully covered by the silica nanoribbons and silicon aggregates responsible for the emission is not as sensitive to the irradiation as the OVCs. The intensity of the red emission increased with the irradiation time from 0 s to about 600 s; and the UV emission first increased and then decreased within the first 300 s. In the case of the blue emission, the intensity escalated rapidly to a constant intensity within the first 240 s. With irradiation, green emission was being overlapped by the strong blue emission. The trend of the irradiation effect on the luminescence of the nanoribbons is similar to the previous observations on amorphous SiO 2 films 41 . Nevertheless, the previous amorphous SiO 2 films 41 did not possess the green emission as detected on our substrate, and we also failed to observe the blue emission from the substrate. The different behaviours of the UV and blue emissions upon the electron irradiation indicate different origins of the emissions. These results indicate that the electron irradiation significantly increases the number of luminescent centers of the silica nanoribbons, strongly dependent on their stoichiometries or absolute deficiency of oxygen atoms over silicon atoms 47,48 .
In summary, we produced silica nanoribbons by a facile method. The silica nanoribbons look like chlorophytum comosum. The silica nanoribbons have a strongly characteristic blue luminescence at about 476 nm as compared to the SiO 2 substrate. Moreover, the silica nanoribbons are flexible and can be manipulated by electron-beam. Whatever the defects responsible for the emissions, this new structure of silica enriches our knowledge of silica-based materials. While the allotropes of bulk silica have made a great impact in fundamental and applied research, the exceptional properties revealed here herald that the grassy silica nanoribbons may present potential applications for, such as composite materials and solar cells as antireflection coating and radiative cooling. Methods Silica Nanoribbon Synthesis. We used chemical vapor deposition method to synthesize the silica nanoribbons. SiO 2 /Si substrate coated with a thin layer of Cr (99.95%) was used for silica nanoribbon growth under a sulfur atmosphere. Vapor of sulfur was produced by heating surfur powder. The growth temperature was about 820 °C. We tried to get crystalline silica by annealing the as-prepared silica nanoribbons at 1000 °C for 2 h under Ar atmosphere.

Materials characterization.
Characterizations of the silica nanoribbons were carried out by using an atomic force microscope (AFM, Digital Instrument Nanoscope IIIA), a powder X-ray diffractometer (XRD, Rigaku D/Max Ultima IV), a field emission scanning electron microscope (FESEM, Hitachi S-4800), a transmission electron microscope (TEM, FEI TECNAIG2 F20-TWIN) equipped with an energy dispersive X-ray spectroscope (EDS), and X-ray photoelectron spectroscopy (ESCALAB 250Xi system). The topographic images of the silica nanoribbons were acquired in the tapping mode. The SEM images were obtained on a Hitachi S4800 field-emission SEM system with an accelerating voltage of 3.0 kV. The XRD was operated at 40 kV, 40 mA for Cu Kα radiation (λ = 1.5418 Å). XPS was performed using Al Kα as the source and the C 1s peak at 284.8 eV as an internal standard. An acceleration voltage of 200 kV was used for EDS measurement. Raman spectrum was obtained by using Renishaw inVia system with a 532 nm laser in ambinet air. Fourier transform infrared spectroscopy (FTIR) was recorded on a Bruker Vector 22 spectrofluorometer in ambient air.
Cathodoluminescence (CL) Characterization. CL measurement was performed by using field emission scanning electron microscope (FESEM;HITACHI SU6600) equipped with CL system (HORIBA MP32). An acceleration voltage of 5.0 kV and beam current of 0.1 nA were used for CL measurement.
Scanning Auger Electron Spectroscopy. The AES measurements were performed at room temperature with a scanning Auger electron spectroscope (ULVAC-PHI model SAM650) with a cylindrical mirror analyzer. The takeoff angle of the instrument was 42°. AES spectra were acquired with a primary electron beam of 10 keV. The incident electron beam current for the AES spectra was about 2.0 nA, as calibrated with a Faraday cup before and after each measurement. Area-analysis mode can be chosen to acquire electron spectra. Acquiring of each elemental map (512 × 512 pixels) took ~3.5 h.