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Local super-saturation dependent synthesis of MgO nanosheets
Applied Surface Science 257 (2011) 3607–3611
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Local super-saturation dependent synthesis of MgO nanosheets Luwei Sun, Haiping He ∗ , Chao Liu, Zhizhen Ye State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 15 October 2010 Received in revised form 13 November 2010 Accepted 13 November 2010 Available online 19 November 2010 Keywords: Magnesium oxide Chemical vapor deposition Surface energy Nanostructures Super-saturation
a b s t r a c t Well-crystallized MgO nanosheets have been prepared with MgB2 as a precursor without any catalyst via a simple chemical vapor deposition (CVD) method. The nanosheets are grown parallel to (2 0 0) plane according to the high-resolution transmission electron microscopy profiles. At the same time, MgO nanowires are formed in the different area of substrate, which is the result of the difference in local super-saturation. Consequently, we propose that the growth mechanism depends on the surface energy and the local super-saturation in the system. © 2010 Elsevier B.V. All rights reserved.
1. Introduction A variety of nanostructures have inspired a number of researchers to explore their novel properties and versatile applications [1,2]. Although nano-materials with various shapes have been synthesized, controllable growth of nanostructures still challenges the scientists in physics, chemistry and materials science. The morphology of some nanostructures has been manipulated by adjusting growth conditions. Since the ordered nanostructures are appreciated in promising applications, such as solar cells [3,4], photo-detectors [5], light-emitting diodes [6], fully understanding the growth mechanism of nanostructures is advantageous in overcoming the difficulties on the controllable synthesis. The study of two-dimensional nanostructures is an important branch of nano-material science and technology. All kinds of nanosheets, such as TiO2 [7], MnO2 [8], ZnO [9], NiO [10], ZnS [11], graphene [12], have been prepared by various methods. MgO is a typical insulator with a wide band gap (Eg = 7.7 eV at 298 K) with rocksalt structure. MgO (1 0 0) facet is most commonly exposed and has the lowest surface energy [13]. MgO (1 0 0) plane is the most stable in dry environment, which is caused by a simple orthogonal structure with an alternation of Mg2+ and O2− ions in favor of physical adsorption instead of chemisorption. Thus, the extensive applications of MgO nanostructures in heatresistance, adsorption [14], catalysis [15], superconductors [16],
∗ Corresponding author. E-mail address: [email protected] (H. He). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.11.087
magneto resistance [17], passive layer in transistors [18], etc., are orientation-dependent. In principle, (1 1 1) facet in rocksalt structure is so unstable in dry environment and at low temperature that it has a tendency to be faceted or reconstructed due to an electrostatic dipole perpendicular to the surface. The stability of MgO facets has been studied both theoretically and experimentally [13], but many issues are still controversial. Recently, MgO nanostructures with various desired shapes, such as nanotubes [19], nanobelts [20], nanowires [21], nanoflowers [22,23] and nanocrystals [24], have been obtained. Although a few reports on MgO nanosheets have been published [25], most of them were synthesized in aqueous solution. In this paper, we report MgO nanosheets prepared by a simple chemical vapor deposition (CVD). The crystallographic orientation of the as-obtained MgO nanosheets is identified on the basis of high-resolution transmission electron microscopy (HRTEM) images. It is also found that the products in different area of the substrate show different morphology, which is attributed to different local super-saturation and surface energy. A diagram is proposed to describe the growth process. 2. Experimental details The synthesis was carried out in a horizontal quartz tube with a diameter of 4 cm and a length of 110 cm. A mixture of high purity magnesium boride (MgB2 ) powder, zinc oxide (ZnO) powder and graphite powder, was loaded into the center of an alumina crucible as the source material. All chemicals are purchased from Sigma–Aldrich without any purification. Here, ZnO was expected
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to facilitate the formation of cubic Mgx Zn1−x O alloy. A c-plane sapphire supported by two silicon sticks on the alumina crucible was located above the source powders. A rotary pump kept vacuum at 1 × 10−2 Torr (1.33 Pa) until the termination of the reaction. The system was heated under the protection of argon gas and held at 850 ◦ C for 30 min. After the growth, the furnace was naturally cooled down to the room temperature. Some dark grey powder was deposited in the areas masked by the silicon sticks, while light grey powder was found elsewhere (Fig. 2(a)). The scanning electron microscopy (SEM) examination was performed by Hitachi S-4800 at an acceleration voltage of 5 kV. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy analysis were carried out with JEM-2010 transmission electron microscope with a point resolution of 0.14 nm operating at 200 kV. The X-ray diffraction (XRD) pattern was measured using Rigaku D/max 2550pc test system.
3. Results and discussion Fig. 1. XRD pattern of MgO nanosheets is shown that pentagram stands for diffraction peaks of MgO; solid dot stands for diffraction peaks of ZnO.
The typical XRD pattern of our sample is shown in Fig. 1. The dominant peak at 42.96◦ is attributed to cubic MgO (2 0 0) plane. The intensity of the peak at 62.37◦ indexed to MgO (2 2 0) plane is
Fig. 2. (a) Schematic visualization of as-obtained MgO nanosheets and nanowires in different area on the substrate, respectively, where we only draw the nanowires and nanosheets in order for simplicity and do not represent the blend of nanosheets and nanowires. SEM images of MgO nanosheets at (b) low and (c) high magnification, respectively. (d) The blend of MgO nanosheets and nanowires, (e) and (f) is MgO nanowires at low and high magnification, respectively.
L. Sun et al. / Applied Surface Science 257 (2011) 3607–3611
3 times weaker than that of (2 0 0) plane. The diffraction peaks at 36.95◦ , 74.78◦ and 78.76◦ are designated to (1 1 1), (3 1 1) and (2 2 2) planes in MgO bulk crystals, respectively. The excess ZnO powder and some ZnO nanostructure by-products are responsible for other five weak peaks. The distribution of the as-synthesized MgO nanosheets and nanowires on the substrate is schematically displayed in Fig. 2(a). The MgO nanosheets with a thickness of 80–200 nm and a length of several microns are observed in the top-view SEM images in Fig. 2(b) and (c). It is notable that the side faces of the nanosheets are a bit rough and some steps and kinks are also observed. Interestingly, whiskers with a uniform diameter of ∼100 nm and a length of several microns extend from the corner of the nanosheets. The MgO nanowires less than 100 nm in diameter appear in the area of the substrate without attachment to the silicon sticks, as shown in Fig. 2(e) and (f). The roughness on the surface of the nanowires and the knots along the nanowires are observed. Such rough MgO nanostructures have caused a concern and been used for supporting catalyst for a long time [26]. Additionally, a mixture of MgO nanosheets and nanowires is also observed in Fig. 2(d). The microstructure of the nanosheets is characterized by TEM, HRTEM and selected area electron diffraction (SAED). The TEM images of MgO nanosheets are shown in Fig. 3(a) and (b). Two typical areas in Fig. 3(b) marked as c and d are magnified. The lattice-resolved image in Fig. 3(c) reveals that the orthogonal pat-
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tern with interplanar spacing of 0.21 nm corresponds to cubic MgO (2 0 0) plane spacing, which is further confirmed by the SAED pattern in the inset of Fig. 3(c). Fig. 3(d) shows a different inclined pattern of the whiskers connecting to the body of nanosheets. The lattice spacing of 0.24 nm matches MgO (1 1 1) plane spacing. The SAED pattern of whiskers is shown in the inset of Fig. 3(d). In our observations across the nanosheets, we do not find any other pattern corresponding to ZnO with wurtzite structure. We also conduct the energy dispersive X-ray spectroscopy (EDS) in the mode of area mapping taken from the junction between the nanosheets and the whiskers to detect the distribution of specific element. Fig. 4 indicates that the nanosheets and the whiskers are composed of MgO. The atomic ratio of Mg and O is close to 1:1 (51 at.%:48 at.%), which is consistent with the stoichiometric ratio of MgO bulk materials. Fig. 4(c) shows that about 1 at.% zinc is detected in the nanosheets. The SAED patterns in the inset of Fig. 3(c) and (d) indicates that a few zinc atoms evaporating from ZnO have been incorporated into MgO crystal lattice in favor of the formation of cubic Mgx Zn1−x O alloys. The unintentionally detected pure ZnO nanosheets (not shown) are responsible for the five weak peaks in Fig. 1. The vapor–liquid–solid (VLS) growth mechanism, proposed by Wagner and Ellis [27], has been widely used to guide the growth of nanostructures, in which metal droplet serving as catalyst plays a crucial role in determining the growth direction, size and loca-
Fig. 3. TEM images of MgO nanosheets are shown in (a) and (b) at low-resolution; (c) and (d) are lattice-fringe image of MgO (2 0 0) and (1 1 1) and are enlargement of circled part in (b), respectively. The selected area electron diffraction patterns are also shown in the insets.
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Fig. 4. EDS profiles of selective area element mapping in the MgO nanosheet (a)–(c) are the concentration distribution of Mg, O and Zn element, respectively. (d) EDS sum spectra of MgO nanosheets.
tion. Experimentally, metal particles are patterned on the substrate and the substrate is heated to the temperature above the eutectic point of expected materials and catalyst. On the other hand, many research groups have prepared nanostructures without any catalyst, which is attributed to the vapor–solid (VS) mechanism with first works by Sears [28], who described the anisotropic formation of the whiskers [29]. In our synthesis of MgO nanosheets, MgB2 precursor decomposed as follows [30]: MgB2 → Mg + MgB4
(1)
At high temperature, ZnO powder was simultaneously reduced by graphite powder and the magnesium vapor produced from MgB2 described by expression (1). Meanwhile, carbon monoxide (CO) gas was produced from the graphite to act as oxygen source. The CO gas carried by argon gas reacted with the high concentration magnesium vapor to generate gaseous MgO, and then MgO nanostructures deposited on the substrate. Researchers have revealed that the probability of twodimensional nucleation was described as the following expression [31]:
PN = Bexp
2 − 2 2 k T ln ˛
(2)
where PN is the two-dimensional nucleation probability; is the surface energy of the materials; ˛ is the super-saturation ratio determined by ˛ = p/p0 , where p is the actual pressure in the reaction system; p0 is the vapor pressure at the equilibrium and depends on the temperature (T). The higher Mg vapor pressure benefits to the generation of MgO nanosheets, according to Eq. (2). As shown in Fig. 2(a), the vapor pressure over the substrate masked by the silicon sticks is higher than that in other areas because the local pressure is inversely proportional to the local volume at isothermal condition, leading to the increase of the probability of two-dimensional nucleation. As a result, the nanosheets are observed in such areas. It is empirically considered that crystal growth occurs on the plane with the lowest surface energy. In cubic MgO crystals, (2 0 0) plane has the lowest surface energy of 1.25 J/m2 , less than that of (2 2 0) plane with 3.02 J/m2 and (1 1 1) plane with 3.86 J/m2 (Ref. [13]). It is verified from the corresponding latticeresolution HRTEM image and SAED pattern in Fig. 3(c) that the MgO nanosheets were grown parallel to (2 0 0) plane. The growth process is schematically described in detail in Fig. 5. The Mg atoms arriving at the (2 0 0) plane have a trend of desorption due to lower surface potential barrier, and then aggregate on the edge of the (2 0 0) plane, leading to two-dimensional structure. On the other hand, a lot of dangling bonds with excessive energy adhere to the plane with lower surface energy, resulting in the total system energy decrease.
L. Sun et al. / Applied Surface Science 257 (2011) 3607–3611
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is attributed to a higher local vapor pressure in favor of twodimensional nucleation. Additionally, surface energy plays a crucial role in nucleation, but the atoms or molecules do not certainly stay at the facet with the lowest surface energy in the presence of a competition between adsorption and desorption. We also demonstrate that cubic Mgx Zn1−x O alloy nanostructures can be synthesized by this method, which has potential applications in solar-blind UV photo-detectors. Acknowledgements This work is financially supported by the National Science Foundation of China (60806003) and Foundation of Doctoral Program of Ministry of Education of China (20070335010). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] Fig. 5. The diagram described for the growth of MgO nanosheets and nanowires.
The lower surface energy, namely lower energy barrier, helps the adatoms leaving away from the surface due to higher mobility at higher temperature. These adatoms are once again captured by the dangling bonds to form clusters at the side, indicating that there is equilibrium between the adsorption and desorption on the active plane.
[18]
[19] [20] [21] [22] [23] [24]
4. Conclusions In conclusion, we have synthesized MgO nanosheets by CVD method. The as-obtained MgO nanosheets with good crystallinility are identified to grow parallel to (2 0 0) plane. MgO nanowires with rough surfaces are simultaneously produced in other areas of the same substrate. We propose that the morphology of MgO nanostructures depends on local vapor pressure that is directly related to the super-saturation. The formation of MgO nanosheets
[25] [26] [27] [28] [29] [30] [31]
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Local super-saturation dependent synthesis of MgO nanosheets Luwei Sun, Haiping He ∗ , Chao Liu, Zhizhen Ye State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 15 October 2010 Received in revised form 13 November 2010 Accepted 13 November 2010 Available online 19 November 2010 Keywords: Magnesium oxide Chemical vapor deposition Surface energy Nanostructures Super-saturation
a b s t r a c t Well-crystallized MgO nanosheets have been prepared with MgB2 as a precursor without any catalyst via a simple chemical vapor deposition (CVD) method. The nanosheets are grown parallel to (2 0 0) plane according to the high-resolution transmission electron microscopy profiles. At the same time, MgO nanowires are formed in the different area of substrate, which is the result of the difference in local super-saturation. Consequently, we propose that the growth mechanism depends on the surface energy and the local super-saturation in the system. © 2010 Elsevier B.V. All rights reserved.
1. Introduction A variety of nanostructures have inspired a number of researchers to explore their novel properties and versatile applications [1,2]. Although nano-materials with various shapes have been synthesized, controllable growth of nanostructures still challenges the scientists in physics, chemistry and materials science. The morphology of some nanostructures has been manipulated by adjusting growth conditions. Since the ordered nanostructures are appreciated in promising applications, such as solar cells [3,4], photo-detectors [5], light-emitting diodes [6], fully understanding the growth mechanism of nanostructures is advantageous in overcoming the difficulties on the controllable synthesis. The study of two-dimensional nanostructures is an important branch of nano-material science and technology. All kinds of nanosheets, such as TiO2 [7], MnO2 [8], ZnO [9], NiO [10], ZnS [11], graphene [12], have been prepared by various methods. MgO is a typical insulator with a wide band gap (Eg = 7.7 eV at 298 K) with rocksalt structure. MgO (1 0 0) facet is most commonly exposed and has the lowest surface energy [13]. MgO (1 0 0) plane is the most stable in dry environment, which is caused by a simple orthogonal structure with an alternation of Mg2+ and O2− ions in favor of physical adsorption instead of chemisorption. Thus, the extensive applications of MgO nanostructures in heatresistance, adsorption [14], catalysis [15], superconductors [16],
∗ Corresponding author. E-mail address: [email protected] (H. He). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.11.087
magneto resistance [17], passive layer in transistors [18], etc., are orientation-dependent. In principle, (1 1 1) facet in rocksalt structure is so unstable in dry environment and at low temperature that it has a tendency to be faceted or reconstructed due to an electrostatic dipole perpendicular to the surface. The stability of MgO facets has been studied both theoretically and experimentally [13], but many issues are still controversial. Recently, MgO nanostructures with various desired shapes, such as nanotubes [19], nanobelts [20], nanowires [21], nanoflowers [22,23] and nanocrystals [24], have been obtained. Although a few reports on MgO nanosheets have been published [25], most of them were synthesized in aqueous solution. In this paper, we report MgO nanosheets prepared by a simple chemical vapor deposition (CVD). The crystallographic orientation of the as-obtained MgO nanosheets is identified on the basis of high-resolution transmission electron microscopy (HRTEM) images. It is also found that the products in different area of the substrate show different morphology, which is attributed to different local super-saturation and surface energy. A diagram is proposed to describe the growth process. 2. Experimental details The synthesis was carried out in a horizontal quartz tube with a diameter of 4 cm and a length of 110 cm. A mixture of high purity magnesium boride (MgB2 ) powder, zinc oxide (ZnO) powder and graphite powder, was loaded into the center of an alumina crucible as the source material. All chemicals are purchased from Sigma–Aldrich without any purification. Here, ZnO was expected
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to facilitate the formation of cubic Mgx Zn1−x O alloy. A c-plane sapphire supported by two silicon sticks on the alumina crucible was located above the source powders. A rotary pump kept vacuum at 1 × 10−2 Torr (1.33 Pa) until the termination of the reaction. The system was heated under the protection of argon gas and held at 850 ◦ C for 30 min. After the growth, the furnace was naturally cooled down to the room temperature. Some dark grey powder was deposited in the areas masked by the silicon sticks, while light grey powder was found elsewhere (Fig. 2(a)). The scanning electron microscopy (SEM) examination was performed by Hitachi S-4800 at an acceleration voltage of 5 kV. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy analysis were carried out with JEM-2010 transmission electron microscope with a point resolution of 0.14 nm operating at 200 kV. The X-ray diffraction (XRD) pattern was measured using Rigaku D/max 2550pc test system.
3. Results and discussion Fig. 1. XRD pattern of MgO nanosheets is shown that pentagram stands for diffraction peaks of MgO; solid dot stands for diffraction peaks of ZnO.
The typical XRD pattern of our sample is shown in Fig. 1. The dominant peak at 42.96◦ is attributed to cubic MgO (2 0 0) plane. The intensity of the peak at 62.37◦ indexed to MgO (2 2 0) plane is
Fig. 2. (a) Schematic visualization of as-obtained MgO nanosheets and nanowires in different area on the substrate, respectively, where we only draw the nanowires and nanosheets in order for simplicity and do not represent the blend of nanosheets and nanowires. SEM images of MgO nanosheets at (b) low and (c) high magnification, respectively. (d) The blend of MgO nanosheets and nanowires, (e) and (f) is MgO nanowires at low and high magnification, respectively.
L. Sun et al. / Applied Surface Science 257 (2011) 3607–3611
3 times weaker than that of (2 0 0) plane. The diffraction peaks at 36.95◦ , 74.78◦ and 78.76◦ are designated to (1 1 1), (3 1 1) and (2 2 2) planes in MgO bulk crystals, respectively. The excess ZnO powder and some ZnO nanostructure by-products are responsible for other five weak peaks. The distribution of the as-synthesized MgO nanosheets and nanowires on the substrate is schematically displayed in Fig. 2(a). The MgO nanosheets with a thickness of 80–200 nm and a length of several microns are observed in the top-view SEM images in Fig. 2(b) and (c). It is notable that the side faces of the nanosheets are a bit rough and some steps and kinks are also observed. Interestingly, whiskers with a uniform diameter of ∼100 nm and a length of several microns extend from the corner of the nanosheets. The MgO nanowires less than 100 nm in diameter appear in the area of the substrate without attachment to the silicon sticks, as shown in Fig. 2(e) and (f). The roughness on the surface of the nanowires and the knots along the nanowires are observed. Such rough MgO nanostructures have caused a concern and been used for supporting catalyst for a long time [26]. Additionally, a mixture of MgO nanosheets and nanowires is also observed in Fig. 2(d). The microstructure of the nanosheets is characterized by TEM, HRTEM and selected area electron diffraction (SAED). The TEM images of MgO nanosheets are shown in Fig. 3(a) and (b). Two typical areas in Fig. 3(b) marked as c and d are magnified. The lattice-resolved image in Fig. 3(c) reveals that the orthogonal pat-
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tern with interplanar spacing of 0.21 nm corresponds to cubic MgO (2 0 0) plane spacing, which is further confirmed by the SAED pattern in the inset of Fig. 3(c). Fig. 3(d) shows a different inclined pattern of the whiskers connecting to the body of nanosheets. The lattice spacing of 0.24 nm matches MgO (1 1 1) plane spacing. The SAED pattern of whiskers is shown in the inset of Fig. 3(d). In our observations across the nanosheets, we do not find any other pattern corresponding to ZnO with wurtzite structure. We also conduct the energy dispersive X-ray spectroscopy (EDS) in the mode of area mapping taken from the junction between the nanosheets and the whiskers to detect the distribution of specific element. Fig. 4 indicates that the nanosheets and the whiskers are composed of MgO. The atomic ratio of Mg and O is close to 1:1 (51 at.%:48 at.%), which is consistent with the stoichiometric ratio of MgO bulk materials. Fig. 4(c) shows that about 1 at.% zinc is detected in the nanosheets. The SAED patterns in the inset of Fig. 3(c) and (d) indicates that a few zinc atoms evaporating from ZnO have been incorporated into MgO crystal lattice in favor of the formation of cubic Mgx Zn1−x O alloys. The unintentionally detected pure ZnO nanosheets (not shown) are responsible for the five weak peaks in Fig. 1. The vapor–liquid–solid (VLS) growth mechanism, proposed by Wagner and Ellis [27], has been widely used to guide the growth of nanostructures, in which metal droplet serving as catalyst plays a crucial role in determining the growth direction, size and loca-
Fig. 3. TEM images of MgO nanosheets are shown in (a) and (b) at low-resolution; (c) and (d) are lattice-fringe image of MgO (2 0 0) and (1 1 1) and are enlargement of circled part in (b), respectively. The selected area electron diffraction patterns are also shown in the insets.
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Fig. 4. EDS profiles of selective area element mapping in the MgO nanosheet (a)–(c) are the concentration distribution of Mg, O and Zn element, respectively. (d) EDS sum spectra of MgO nanosheets.
tion. Experimentally, metal particles are patterned on the substrate and the substrate is heated to the temperature above the eutectic point of expected materials and catalyst. On the other hand, many research groups have prepared nanostructures without any catalyst, which is attributed to the vapor–solid (VS) mechanism with first works by Sears [28], who described the anisotropic formation of the whiskers [29]. In our synthesis of MgO nanosheets, MgB2 precursor decomposed as follows [30]: MgB2 → Mg + MgB4
(1)
At high temperature, ZnO powder was simultaneously reduced by graphite powder and the magnesium vapor produced from MgB2 described by expression (1). Meanwhile, carbon monoxide (CO) gas was produced from the graphite to act as oxygen source. The CO gas carried by argon gas reacted with the high concentration magnesium vapor to generate gaseous MgO, and then MgO nanostructures deposited on the substrate. Researchers have revealed that the probability of twodimensional nucleation was described as the following expression [31]:
PN = Bexp
2 − 2 2 k T ln ˛
(2)
where PN is the two-dimensional nucleation probability; is the surface energy of the materials; ˛ is the super-saturation ratio determined by ˛ = p/p0 , where p is the actual pressure in the reaction system; p0 is the vapor pressure at the equilibrium and depends on the temperature (T). The higher Mg vapor pressure benefits to the generation of MgO nanosheets, according to Eq. (2). As shown in Fig. 2(a), the vapor pressure over the substrate masked by the silicon sticks is higher than that in other areas because the local pressure is inversely proportional to the local volume at isothermal condition, leading to the increase of the probability of two-dimensional nucleation. As a result, the nanosheets are observed in such areas. It is empirically considered that crystal growth occurs on the plane with the lowest surface energy. In cubic MgO crystals, (2 0 0) plane has the lowest surface energy of 1.25 J/m2 , less than that of (2 2 0) plane with 3.02 J/m2 and (1 1 1) plane with 3.86 J/m2 (Ref. [13]). It is verified from the corresponding latticeresolution HRTEM image and SAED pattern in Fig. 3(c) that the MgO nanosheets were grown parallel to (2 0 0) plane. The growth process is schematically described in detail in Fig. 5. The Mg atoms arriving at the (2 0 0) plane have a trend of desorption due to lower surface potential barrier, and then aggregate on the edge of the (2 0 0) plane, leading to two-dimensional structure. On the other hand, a lot of dangling bonds with excessive energy adhere to the plane with lower surface energy, resulting in the total system energy decrease.
L. Sun et al. / Applied Surface Science 257 (2011) 3607–3611
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is attributed to a higher local vapor pressure in favor of twodimensional nucleation. Additionally, surface energy plays a crucial role in nucleation, but the atoms or molecules do not certainly stay at the facet with the lowest surface energy in the presence of a competition between adsorption and desorption. We also demonstrate that cubic Mgx Zn1−x O alloy nanostructures can be synthesized by this method, which has potential applications in solar-blind UV photo-detectors. Acknowledgements This work is financially supported by the National Science Foundation of China (60806003) and Foundation of Doctoral Program of Ministry of Education of China (20070335010). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] Fig. 5. The diagram described for the growth of MgO nanosheets and nanowires.
The lower surface energy, namely lower energy barrier, helps the adatoms leaving away from the surface due to higher mobility at higher temperature. These adatoms are once again captured by the dangling bonds to form clusters at the side, indicating that there is equilibrium between the adsorption and desorption on the active plane.
[18]
[19] [20] [21] [22] [23] [24]
4. Conclusions In conclusion, we have synthesized MgO nanosheets by CVD method. The as-obtained MgO nanosheets with good crystallinility are identified to grow parallel to (2 0 0) plane. MgO nanowires with rough surfaces are simultaneously produced in other areas of the same substrate. We propose that the morphology of MgO nanostructures depends on local vapor pressure that is directly related to the super-saturation. The formation of MgO nanosheets
[25] [26] [27] [28] [29] [30] [31]
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