Arrays of One-Dimensional Germanium Cone-Like Nanostructures

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Arrays of One-Dimensional Germanium Cone-Like Nanostructures: Preparation and Application as Fluorescent pH Sensor Yunyu Liu,†,‡ Rong Miao,†,‡ Guangwei She,† Lixuan Mu,† Yao Wang,†,‡ and Wensheng Shi*,† †

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Graduate School of Chinese Academy of Sciences, Beijing 100039, China ABSTRACT: Highly oriented arrays of one-dimensional Ge cone-like nanostructures were prepared in the aqueous solution of HF and H2O2 by a facile chemical etching method. The morphologies of the cone-like nanostructure arrays can be readily controlled by modulating the compositions, temperature, and concentrations of the etching solution as well as the etching duration. It is suggested that the positive charged holes play a critical role in the formation of the present cone-like nanostructures. By covalently bonding the fluorescein derivatives onto the surfaces of the Ge nanostructures, a fluorescent pH sensor with a good sensitivity and selectivity was achieved.

1. INTRODUCTION Ge is an important Group 14 element semiconductor with a narrow band gap of 0.66 eV, which promises potential applications in infrared optical devices1,2 and high-efficiency solar cells.3,4 Moreover, due to the high electron and hole mobility, Ge could be utilized to fabricate high-frequency devices.5
8 Recently, micro-/nanostructures have attracted considerable attention for their novel properties and their wide potential applications. Accordingly, several methods have been developed to fabricate one-dimensional (1D) Ge nanostructures. The hydrothermal method,9 laser ablation,10,11 and chemical vapor deposition 12 have been used; however, the obtained Ge nanostructures are orientational disordered. Meanwhile, the ordered arrays of the Ge nanowires have been fabricated by chemical vapor deposition (CVD),13
17 electrochemical etching,6 and the template-assisted supercritical fluid method,18 which mostly involve severe experimental conditions, complex processes, or expensive equipment. Sometimes, a metal catalyst has to be used during these processes, and as a result, the metal impurities are unavoidably introduced in the products. Chemical etching is a simple and quite effective way to fabricate nanostructures, such as high ordered arrays of Si nanowire19,20 and GaN nanostructures.21,22 In this study, a simple chemical etching method was developed to prepare the highly oriented arrays of the 1D Ge/GeOx core/shell structures. Moreover, by a postannealing treatment in the reducing ambient, 1D cone-like nanostructure arrays of the pure Ge can be obtained.23 It was found that the morphologies of the present 1D Ge nanostructures could be readily controlled by modulating the compositions, temperature, and concentrations of the etching solution and the etching duration. Furthermore, based on the 1D Ge nanostructure arrays, a sensitive fluorescent pH sensor was r 2011 American Chemical Society

achieved by covalently bonding the fluorescein derivatives onto the surface of the 1D Ge cone-like nanostructures.

2. EXPERIMENTAL SECTION The p-type (100) and n-type (100) single crystalline Ge wafers with the size of 2 mm  4 mm were washed ultrasonically with ethanol and acetone and then rinsed with deionized water. The etching process was carried out in 30 mL polyethylene centrifuge tubes. The mixed aqueous solution of HF and H2O2 was used as the etchant, and the concentrations of HF and H2O2 were varied from 4.8 to 10 M and 0.1 to 0.2 M, respectively. The centrifuge tubes were filled with 20 mL of etchant and kept at 20
50 °C. Then, the cleaned Ge wafers were placed into the etchant. After certain etching durations, the wafers were taken out, rinsed with distilled water, and dried under ambient, room temperature conditions. Finally, the etched Ge wafers were annealed at 550 °C in a reducing ambient (Ar:H2 = 95:5). The fluorescent pH sensor was prepared by the process as follows: the postannealed samples were first cleaned ultrasonically in ethanol for 10 min to remove the organic contaminants from the surface and then blow-dried with nitrogen. To provide abundant reactive sites at the surfaces of the samples for bonding with the silane in the following step, the cleaned samples were bombarded by the oxygen plasma with 25% oxygen at 500 V for 5 min. After this treatment, the samples were immediately immersed into 10 mM 3-aminopropyltriethoxysilane (APTES) methylbenzene solution in a round-bottom flask and maintained at 90 °C for 12 h. After cooling to room temperature, Received: June 21, 2011 Revised: September 26, 2011 Published: September 28, 2011 21599

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Figure 1. SEM images of the 1D structure arrays etched with p-type (100) Ge substrates in 4.8 M HF and 0.2 M H2O2 (a) at 20 °C and (b) at 50 °C.

the APTES-modified samples were repeatedly rinsed with ethanol to remove the unreacted APTES from the samples and subsequently soaked in 50% glutaraldehyde (aqueous solution) for 2 h at room temperature. Repeatedly washing with ethanol, the samples were then immersed in 2 mM fluoresceinamine (ethanol solution) for 1.5 h at room temperature. Finally, the samples were soaked in 3 mM sodium triacetoxyborohydride (ethanol solution) and maintained at 50 °C for 2 h. After washing with ethanol, the pH sensors were obtained. The morphologies of the products were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were employed to determine the compositions of the samples. A fluorescence microscope with a 100 W mercury lamp was used to directly observe the fluorescence intensities of the sensors under different pH values. The excitation wavelength was modulated at 470
495 nm, and the fluorescent microscope pictures were recorded under 100 magnifications.

3. RESULTS AND DISCUSSION When the p-type (100) Ge wafers were etched in an aqueous solution containing 4.8 M HF and 0.2 M H2O2 at 20 °C, ordered arrays of column-like nanostructures were obtained. The columns with a diameter of 0.5
10 μm and a length of about 50 μm are almost perpendicular to the substrate (Figure 1a). It was found that the morphologies of the products depended on the etching temperature. By increasing the temperature to 50 °C, ordered arrays of the cone-like nanostructures can be obtained (Figure 1b). The length of the cones is about 10 μm, which is much shorter than that of the nanocolumns. To determine the chemical compositions of the products, the as-prepared samples were investigated by EDS and XPS. Figure 2a is a typical TEM image of a single nanocone. The EDS analysis indicates that the cone contains only Ge and O, and the O contents at different parts of the cone are quite different.

Figure 2. (a) TEM image of a single cone. The atom ratios O/Ge at different locations of the cone are in the inset. (b) High-resolution Ge 3d XPS of 1D cone nanostructure arrays and the three fitting lines of Ge0, Ge2+, and Ge4+. The vertical lines indicate the center position of Ge2+(GeO), Ge4+(GeO2), and Ge0(Ge).

The variation of the O content in a single cone shown in Figure 2a was investigated by the EDS attached to TEM. The atomic ratios of the O/Ge at different positions of the cone are shown in the table inset in Figure 2a. These results reveal that the cones are composed of Ge and O, and the oxidation is not uniform in the whole cone. The O content is highest at the tip of the cone. From the tip to the bottom of the cone, the O content decreases gradually, and finally no O could be detected. The products were further characterized by XPS to confirm the chemical composition and chemical state. Figure 2b shows the Ge 3d XPS taken from the cone arrays. The Ge 3d peak can be separated into three peaks at 32.5, 30.5, and 29.4 eV, corresponding to GeO2, GeO, and Ge, respectively.24,25 The results demonstrate that the as-etched samples consist of Ge and GeOx (x = 1, 2). The EDS and XPS analysis results of the columns are almost the same. As the GeO2 and GeO can be dissolved in the HF solution,26
28 the samples were immersed into the HF solution with a concentration of 40% for 4 h to confirm their structures. The SEM images of the sample before (inset image) and after the immersion are shown in Figure 3a. It can be seen that the diameter and the length of the columns were reduced after the HF treatment, indicating that the GeOx exists in the outside layer of the columns. Considering all of the above results, it can be demonstrated that the as-prepared sample is a Ge/GeOx core/shell structure. 21600

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Moreover, as the GeOx can be converted to Ge by the reduction process,23 a postannealing treatment was performed in the reducing ambient at 550 °C for 4 h. After annealing, no O element could be detected by the EDS, and immersing the annealed sample into HF solution will not change its morphology (Figure 3 b). This result verifies that the samples after the annealing process are the pure Ge nanostructures. The formation of the Ge/GeOx 1D nanostructure arrays can be understood as the following chemical etching process. When dipped into the etching solution, the surface of the Ge wafer was oxidized by H2O2 to form Ge(OH)2 and GeO2 (eqs 1 and 2). The Ge(OH)2 can be subsequently oxidized by H2O2 to form GeO2 (eq 3). GeO2 is slightly soluble in the etching solution (eq 4).23 The reactions are summarized as follows Ge þ H2 O2 f GeðOHÞ2

ð1Þ

Ge þ 2H2 O2 f GeO2 þ 2H2 O

ð2Þ

GeðOHÞ2 þ H2 O2 f GeO2 þ 2H2 O

ð3Þ

GeO2 þ 6HF f H2 GeF6 þ 2H2 O

ð4Þ

Simultaneously, the naked Ge can also be directly etched by HF (eq 5) Ge þ 6F
þ 4hþ f GeF6 2


Figure 3. (a) SEM image of the unannealed sample soaked in 40% HF for 4 h (inset: the same sample before soaking in HF). (b) SEM image of the annealed sample soaked in 40% HF for 4 h (inset: the same sample before soaking in HF).

ð5Þ

By the reaction of eq 5, some grooves were created randomly in the initial stage, and many islands surrounded by these grooves were formed (Figure 4a). It is known that the bulk Ge and the H2O2 (H2O2 + 2H+ f 2H2O + 2h+) can provide the positive charged holes. Considering the random diffusion of the holes within the bulk Ge, the bottom of the grooves geometrically surrounded by the substrate can obtain more holes from not only the surroundings but also H2O2. However, the top of the islands got the holes only from the underside since the holes produced

Figure 4. Etching stage of p-type (100) Ge substrates with 4.8 M HF and 0.1 M H2O2: (a) 1 h; (b) 3 h. (c), (d) Schematics of the etching stage corresponds to (a) and (b), respectively. The black color represents Ge, and the grids represent GeOx. 21601

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Figure 6. Integrated fluorescence intensity of the sensor sample over the fluorescence microscopy images at different pH (from 4 to 9). A and B represent the relative FL intensity recorded at the first test and the test after a week, respectively. Each data point represents the average of three independent measurements, where the error bars are the standard deviation of the mean value. The insets are the fluorescent microscope pictures recorded at pH = 4 and pH = 9, respectively.

Figure 5. (a) n-type (100) Ge was etched with 0.1 M H2O2 and 7 M HF. (b) n-type (100) Ge was etched with 0.1 M H2O2 and 10 M HF. (c) p-type (100) Ge precoated with Ag nanoparticles was etched with 4.6 M HF and 0.2 M H2O2.

by H2O2 were insulated by GeOx at the top of the islands. Promoted by the holes (eq 5), the etching of the grooves is fast, while the etching at the tops of the islands is slow. As a result, the deep grooves were developed. In the meanwhile, GeOx was formed on the surface of the islands by oxidizing Ge (eqs 1, 2, and 3), and the dissolution of GeOx was supervened slowly by HF.29,30 With the development of downward etching on the grooves, continuously oxidizing and dissolving at the surface of the islands would make the final top smaller than the initial one (Figure 4b). These processes can be illustrated by Figure 4c and 4d. Under the same concentration of H2O2, the higher the concentration of HF is, the sharper and longer the final cones would be. Figure 5a and b presents two kinds of 1D Ge nanostructures that were obtained by etching the n-type Ge in HF of 7 and 10 M, respectively. The observation from Figure 5a and 5b is consistent with the above analysis. The ability to supply holes in p-type Ge is higher than that of the n-type one. As a result, the etching on the grooves of the p-type wafer would be fast due to its high concentration of holes. Comparing Figure 1a with Figure 5a, it can be found that the 1D structure arrays etched from the p-type wafer are longer than that using the n-type one, although the former was etched in 5 M HF and the latter in 7 M HF. Accordingly, the etching process would be inhibited if the supply of the holes was blocked. To verify it, a thin layer of Ag

nanoparticles (AgNPs) was precoated on the Ge wafer before etching.19,20 After the AgNP-seeded p-type Ge wafer was etched in the solution of 4.6 M HF and 0.2 M H2O2 for 6 h, the 1D Ge nanostructure with the AgNPs on its tip was obtained as shown in Figure 5c. Since the work function of Ag (4.6 eV) is less than that of Ge (∼5.0 eV),31 the electrons within Ag would be transferred to Ge where the AgNPs were attached. Resultingly, a hole depletion layer would be formed within the Ge close to the interface between AgNPs and Ge. The etching of Ge near the AgNPs would be inhibited by the hole depletion layer, while those places without the AgNPs could be etched easily because the holes were obtainable there. Finally, the arrays of the 1D Ge structure were formed. It can be found that the AgNPs play a mask role due to the critical function of the holes during the etching process. However, very different from the situation of the Ag-assisted Si etching where the AgNPs were involved in a similar electrochemical fuel and moved downward,20 present AgNPs are used to supply electrons and form the hole depletion layer in Ge, which would prevent the etching reaction from occurring beneath the AgNPs. Finally the AgNPs will leave at the top of the 1D Ge structures. These results are consistent with the observation from Figure 5c and further demonstrate that the holes play a critical role in the formation of the Ge 1D structures. Considering the high stability and nontoxicity of the Ge materials, the cone-like nanostructures have potential applications in the detection of cells. The deoxidized Ge 1D nanostructure arrays were utilized to construct a fluorescent pH sensor by covalently decorating the fluorescein onto the surface of the arrays.32 As shown in Figure 6, when the pH is varied from 4 to 9, the fluorescence intensities gradually increase with a good linear relationship between the fluorescence intensities and the pH values. This sensor is based on the fluorescein, thus its selectivity and sensitivity also seriously rely on the properties of the fluorescein modified on the surface of the Ge nanostructure. With regard to the selectivity, the influence of the common interfering ions in the human body was investigated, including K+, Na+, Ca2+, and Cl
. The concentrations of these ions were all higher than that in vivo. Consistent with our expectation, the fluorescence intensity of the sensor was not as affected by these 21602

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The Journal of Physical Chemistry C interfering ions as that of the fluorescein in solution. On the other hand, the present sensor owned an adequate sensitivity. The fluorescence intensity increases by about 3% when pH increases 0.1 near pH = 7 and decreases by about 51% from pH 9 to 5. It was reported that the green and red emission from a luminescent lanthanide nanorod-based pH sensor, respectively, decreased by about 28 and 52% from pH 10 to 6.33 Gao et al. reported core
shell fluorescent silica nanoparticles for sensing nearneutral pH values and achieve a better sensitivity, but the linear region was relatively narrow.34 Moreover, the present sensor is capable of fixing cells and sensing the pH simultaneously and is highly reproducible. The stability and reproducibility have been well verified. After being stored in the air at room temperature for a week, the pH sensor exhibited a small decay (less than 10%) in the fluorescence intensity, and the results are shown in Figure 6. This stability of the present sensor is appropriate for most of the detections. Meanwhile, the as-prepared pH sensor showed a good reproducibility. After the sensors were washed repeatedly with distilled water, the fluorescence intensity showed no obvious change at the same pH. From the fluorescence microscope images inserted in Figure 6, it can be observed that the present sensor owns a high spatiotemporal resolution. As the linear relationship covers the pH of cells in all states, it would be a rational strategy to detect the pH of the cells in situ by culturing the cells onto such stable and nontoxic arrays of the modified Ge cones.

4. CONCLUSIONS Highly oriented arrays of the 1D Ge/GeOx core/shell structure were prepared by a chemically etching process. Annealed in reducing ambient, the samples can be converted to pure Ge 1D nanostructure arrays. By modulating the concentration, composition, and temperature of the etchant, the morphologies of the as-prepared products can be readily controlled. It was found that the holes play a critical role in the formation of the Ge 1D structures. The hole-assisted etching method would provide a possible way to fabricate the 1D nanostructures of other materials. On the basis of the as-prepared 1D Ge nanostructure arrays, a fluorescent sensor was fabricated to sensitively detect the pH value, which promises the present sensor potential applications in the in situ observation of the status of the cells. ’ AUTHOR INFORMATION Corresponding Author

*Tel./Fax: 86-10-82543513. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Chinese Academy of Sciences, NSFC (Grant Nos. 10874189, 50902134, and 61025003), National Basic Research Program of China (973 Program) (Grant Nos. 2007CB936001, 2009CB623703, and 2010CB934103), and Beijing Natural Science Foundation (Grant No. 2102043).

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