Figure 1 shows typical SEM images of the results obtained at a temperature of 1,000°C. The typical overall view of the obtained sample is shown in Figure 1a, where the integrated nanostructures are obviously observed on carbon posts at the downstream part, and the nanostructure density decreased gradually along the carrier gas flow direction. In general, the growth area can be divided into three parts for three different types of morphologies with evolution along with the concentration gradient flow. SEM images of the representative structures from each part are presented in Figure 1b,c,d, and their enlarged SEM images are presented in Figure 1e,f,g, respectively. In Figure 1e, rope-like nanowire curved assemblies with length up to hundreds of micrometers are observed on the carbon post. In Figure 1f, feather-like nanowire assemblies with abundant branches are observed, while octopus-like nanowire assemblies with abundant branches are observed in Figure 1f. Cu catalyst particles are always found at the tip of the assemblies, which could be explained by VLS mechanism 26 . It shows that Si vapor source concentration gradient at microscale has great effect on the nanowire growth rate and the morphology evolution. With the Si vapor flowing from the upstream to the downstream at the designed sample, rope-like structures are grown at the region with the highest Si vapor source concentration, octopus-like ones are grown at the region with the lowest Si vapor source concentration, and feather-like ones are observed at the middle region. Higher Si vapor source concentration leads to a much higher growth rate and a larger amount of nanostructures, and the gradual decrease of Si vapor source concentration could lead to the evolved change of the morphology for grown nanostructures.

TEM images of the nanowire assemblies were shown in Figure 2. Figure 2a shows part of the typical ultra-long rope-like nanowire assemblies with different thicknesses. As shown in Figure 2b, nanowire branching behavior could be observed close to the catalyst tip, where a nanowire with a diameter of 200 nm was split into two branches with diameters of 150 and 300 nm, respectively, during the growth process. Multi-branching and connecting led to thicker ropelike assemblies as shown in Figure 2c. The branching behavior could be explained by VQS mechanism, in which catalyst particles in the presence of Si (silica) may undergo melting even at a temperature lower than the metal/Si eutectic temperature and can thus serve as droplets for the growth of the nanostructure branches 28 29 . A typical TEM image of feather-like nanowire assemblies is shown in Figure 2d, showing the length of about 7 μm and width of about 4 μm. It is also revealed that the feather-like structure has symmetrical features, and a small broken part of the assemblies indicated with arrow shows branching or splitting behaviors of the nanowire assemblies. The branching behavior is further confirmed by the TEM image of a large fragment of the feather-like nanowire assemblies shown in Figure 2e, where the great increase on the amount and volume of the nanowires along the growth direction is due to splitting growth 30 . A typical TEM image of octopus-like nanowire assemblies is shown in Figure 2f, implying that many nanowires grew simultaneously on one catalyst particle while several branching behaviors are also observed. The similar phenomenon of nanowires growing from one catalyst was also reported by KM Sunkara and associates as early as 2001 31 , and it could rely on the cause that the fragmentation of one catalyst particle makes it become many smaller catalyst particles under the influence of thermodynamic imbalance and surface energy 32 . Figure 2g shows the typical HRTEM image of one nanowire without any crystalline domains, indicating that the nanowire is amorphous, which is confirmed by the selected area electron diffraction (SAED) pattern with only diffusive rings instead of diffraction spots, shown as an inset in Figure 2g. Optical devices will have more functions on each chip if silica nanostructures with different morphologies originating from silica nanowires that are integrated 11 13 14 .

Figure 3 shows typical SEM images of nanostructure-integrated carbon posts obtained at the temperature of 950°C with concentration gradient of Si vapor source. The overall view of the sample is shown in Figure 3a, where the integrated nanostructures are obviously observed and the nanostructure density decreased gradually along the carrier gas flow direction. In general, the growth area can be also divided into three parts. SEM images of the representative structures from each part are presented in Figure 3b,c,d, respectively. In Figure 3b, stringlike nanowire assemblies with the length up to hundreds of micrometers are observed. In Figure 3c, bamboo-like structures are observed while a few stringlike nanowire assemblies could also be observed. In Figure 3d, only bamboo-like structures are observed. In general, the concentration gradient of the Si vapor source also affects the formation of nanostructure morphology to cause a slight evolution at 950°C, and the dominant mechanism still follows the VLS.

Unique TEM images of the nanostructure and assemblies from Figure 3b,c regions were shown in Figure 4. Figure 4a shows the typical morphology of a stringlike nanowire assembly with curly features. Figure 4b shows the enlarged view of the head part of the assembly with catalyst tip, indicating that many tiny nanowires grew from the catalyst particle with branching and connecting behaviors 12 . Typical bamboo-like structures with large bowl-like, cuplike, and vase-like joints are observed and shown in Figure 4c,d,e, respectively, which were seldom observed from the Figure 3d region. In Figure 4c, a bamboo-like structure is with bowl-like joints having an outer wall diameter of about 400 nm and a wall thickness of about 50 nm. In Figure 4d,e, the length and diameter of the joint are at a microscale. HRTEM image of the gap between the microscale joints is shown in Figure 4f, and it shows the obvious hollow feature with the outer wall composed of several layers, and the inner wall could be composed of aligned nanowires 20 . The gabs further indicate the batch growth for the bamboo-like structures 30 . Furthermore, all the large joints and the gabs implied that the higher concentration has a great impact on the diverse morphologies of the silica structures to cause the evolution behavior.

Silica nanostructures grown at the temperature of 950°C are shown more in Figure 5, where three types of hollow structures are observed as shown in Figure 5a,b,c, respectively. As shown in Figure 5a, the tubelike structure has an outer diameter of about 450 nm, a length up to 15 μm, and a wall thickness of about 50 nm. A bamboo-like structure with bowl-like joints is shown in Figure 3b, having an outer diameter of about 400 nm, a length up to 10 μm, and a thickness of about 50 nm. Both of their outer and inner diameters increase continuously from the catalyst tip to the larger end, which causes the wire to have a uniform wall thickness. Another type of bamboo-like structure with dumbbell-like joints is shown in Figure 5c, having an uneven wall thickness along the wire. It is distinctively observed that for bowl-like or dumbbell-like joints, the distance between two adjacent ones is rather uniform, ranging from 100 to 150 nm. The bowl-like and dumbbell-like joints indicate batch growth behavior of the nanostructures 30 . The HRTEM in Figure 5d,e shows the features of the inner and outer walls in the hollow structures. The inner walls are essentially composed of chain-like aligned nanowires, while the outer walls are tubular sheaths, which are quite similar to an earlier literature report 20 . The outer walls composed of several layers can be distinctively observed in this study as shown in Figure 5d,f. The HRTEM also indicates the amorphous features of the nanowires confirmed by the SAED in the inset in Figure 5d. A typical EDX analysis confirms that the nanostructure is composed of Si and O with an O/Si atomic ratio of 1.8 as shown in Figure 5g, while the catalyst tips are Cu-rich particles as shown in the EDX analysis of Figure 5h, where the Ni signals are generated from the nickel grid supporting the nanostructures. The performance of biomedical devices involved with tubelike silica would be improved with more controllable features when the silica nanostructures with different tubelike morphologies could be applied in a controllable way 9 10 .

In general, Si vapor source is mainly from the active oxidation of the silicon substrate in the presence of copper at high temperature, and O vapor source might come from the leakage of the vacuum system 26 . The copper layer breaks to small droplets at the heating process, then Cu-Si eutectic catalysts can be formed to initiate the nanostructure growth on the 3D carbon posts above the eutectic temperature, which matches with the photoresist carbonization process 26 . By dissolving Si and O source onto the catalyst droplet continuously, the silica nanostructures grow mainly through VLS and VQS mechanisms. The catalyst droplets with smoothly curved surfaces confirm the VLS process, as shown in Figures 2, 4, and 5 33 . Combining the results of the SEM and TEM, it is concluded that the Si vapor source concentration gradient would lead to unique morphology changes of nanostructures. The underlying reason could be due to the variations of reactive source concentration gradient inside different catalyst droplets caused by the local source concentration gradient in the environment, which is a key factor to determine the nanowire growth behavior and could result in diverse morphologies 32 34 . Furthermore, at a relatively lower temperature of 950°C, the active smaller catalysts in each catalyst might be concentric to initiate tubelike structures due to thermodynamic imbalance, which could be confirmed by Figure 4f 32 .

It is also noticed that silica nanostructures are hard to observe at an annealing temperature below 900°C, while abundant silica nanowires are found all over the substrate when the temperature is above 1,050°C, which reminds us that temperature is another critical parameter to determine the growth behavior of silica nanostructures.