J. Phys. Chem. C 2009, 113, 5201–5205
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Rapid Fabrication of Nanocrystals through in situ Electron Beam Irradiation in a Transmission Electron Microscope Junqing Hu,* Yangang Sun, and Zhigang Chen State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua UniVersity, Shanghai 201620, China ReceiVed: January 10, 2009; ReVised Manuscript ReceiVed: February 5, 2009
In situ electron-beam irradiation was performed in a transmission electron microscope to fabricate nanocrystals of IV-VI semiconducting lead chalcogenides PbE (E ) S, Se, and Te) and low-melting-point metals (Zn, Ga, Sn, and Cd). The source powders consisting particles of 300-1000 nm in diameter were supported on a thin carbon (C) film. A convergent electron beam was focused on an individual particle, which caused partial melting and evaporation of the particle and subsequent nucleation and growth of new nanoparticles on the C film. These nanocrystals are structurally uniform and free of any linear and planar defect and do not have any other impurity contamination. The method is easy to set up and is low in cost and is very fast (several seconds to several minutes) in process. It is believed that this new method can also be employed to synthesize many other material nanocrystals and one-dimensional nanostructures, beyond semiconductors and (low melting point) metals, such as oxides and even sulfides and nitrides. 1. Introduction The development of semiconducting and metallic nanocrystals has been intensively pursued, not only for their fundamental scientific interest but also for many technological applications,1-3 such as in photography,4 catalysis,5 biological labeling,6,7 photonics,8,9 optoelectronics,10 information storage,11 and formulation of magnetic ferrofluids.12 In principle, the intrinsic properties of these semiconducting and metallic nanocrystals are mainly determined by their size, shape, composition, crystallinity, and structure. By controlling one or more of these parameters, the properties of these nanoparticles can be finely tuned.13 With bondings formed through electrostatic interactions among the ions of the crystal lattice,14 IV-VI lead chalcogenides PbE (E ) S, Se, and Te) are semiconducting with good grade of polarity and possess nontypical electronic and transport properties, such as high carrier mobilities, high dielectric constants, narrow band gaps, and positive temperature coefficients.15,16 These properties make them particularly useful as electro-optical devices in the range of 3-30 µm, corresponding to the medium and far-infrared, which will lead to many different technological applications.17 Consequently, considerable progress based on chemical solution route has been made in the synthesis of IV-VI semiconducting nanocrystals.18-20 For metals such as Zn, Cd, Ga, In, and Sn (melting points of 419.6, 321.1, 29.8, 156.6, and 232 °C, respectively), due to their low melting points and high reductivity, the resulting metallic nanoparticles, prepared by routine chemical and physical methods, tend to grow into large (micrometer or even millimeter) sized particles or sheets,21-24 and usually there is an oxide layer on the nanocrystal’s surface. In contrast to other metallic material systems, the challenge of synthetically controlling the shape of these low-melting-point metallic nanoparticles has been only met with limited success. It has been demonstrated that the electron beam generated within transmission electron microscopy (TEM) is very powerful * To whom correspondence should be addressed. E-mail: hu.junqing@ dhu.edu.cn.
for fabrication and manipulation of nanostructures.25-30 These studies, the majority of which involved only pure elements or alloys, showed that local atoms of a small particle undergo collisions and displacements with high-energy electrons.27,28 In this study, we demonstrate that convergent electron beam irradiation (EBI) in TEM can be used effectively for the fabrication of IV-VI compound semiconducting lead chalcogenides PbE (E ) S, Se, and Te) and low-melting-point metallic (Zn, Cd, Ga, In, Sn) nanocrystals. 2. Experimental Section In our experiment, commercial PbE and low melting-pointmetal powders were directly used as the source materials. The PbE or metal powders were placed in a test tube and dispersed in ethanol using an ultrasonic cleaner. After the test tube was removed from the cleaner an eyedropper was used to collect the topmost liquid that mainly contained smaller PbE or metal particles (300-1000 nm in diameter). The collected liquid was then deposited onto a 200-mesh ultrathin holey C film. A JEOL 200 kV (JEM-2100F) field-emission analytical high-resolution TEM, equipped with an X-ray energy dispersive spectrometer (EDS), was used for in situ EBI and characterization. During the typical EBI process, when a magnification of 4000 and a second condenser lens aperture were used, an electron current density onto a TEM full screen was varied in the range of ∼68.0-79.0 pA/cm2. The electron current density of the convergent beam used for irradiation on a specimen is in the range of ∼125-165 pA/cm2, which can be varied by changing a beam size and its brightness. The basic pressure of the TEM chamber during the growth is as low as ∼ 1 × 10-5 Pa. 3. Results and Discussions Figure 1a is a TEM image of the PbS nanoparticles prepared by EBI of PbS powders. The product consists many PbS nanocubes covering the carbon film on the TEM mesh. A high magnification TEM image of the PbS is shown in Figure 1b, which shows that the cubes have smooth faces with a mean
10.1021/jp900247e CCC: $40.75 2009 American Chemical Society Published on Web 03/11/2009
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Figure 1. TEM results of the PbS nanocubes. (a) Low-magnification TEM micrograph. (b) High-magnification TEM micrograph. (c) Selected area ED pattern of the cubes. (d) HRTEM image of an individual PbS nanocube taken with none of its surfaces parallel or perpendicular to the electron beam. (e) HRTEM image of an individual PbS nanocube taken with the electron beam perpendicular to the top surface of the cube. (f) Digital Fourier transform of the HRTEM image shown in Figure 1e. (g) EDS spectrum recorded from a single PbS nanocrystal.
edge length ranging from 10 to 15 nm. A selected area electron diffraction (ED) pattern taken from many PbS cubes is shown in Figure 1c. The first six electron diffraction rings (from the one with the smallest diameter) shown in Figure 1c are in well agreement with the (111), (200), (220), (311), (222), and (400), respectively, atomic plane spacings of the face-centered cubic (fcc) PbS structure (JCPDS card no. 05-0592, space group Fm3jm, a ) 5.9362 Å). Figure 1d shows a high-resolution TEM (HRTEM) image of a PbS cube, which was taken when the sample was slightly tilted so that the incident electron beam (e-) is not perpendicular to the cube’s top surface. It demonstrates that all surfaces and edges are homogeneous and uniform at atomic level. The imaging orientation is close to {211}. Figure 1e shows the HRTEM image of another PbS cube which has its top and bottom surfaces perpendicular to the electron beam. The spacing of the 2D lattice fringes shown in this image was measured to be 0.297 nm, close to the {200} atomic plane spacing of the fcc PbS crystal. Figure 1f shows the Fourier transform of the HRTEM image shown in Figure 1e. This pattern can be readily indexed as the electron diffraction of the fcc PbS in {100} zone axis. Further studies suggest this faceted PbS nanocube is enclosed by six {100} crystal planes. EDS analysis, Figure 1g, suggests that the cubes are made up Pb and S with a Pb/S atomic ratio of ∼1.02 (Cu signals originate
Figure 2. TEM and illustrative results of the PbSe nanocubes. (a) Lowmagnification TEM micrograph. (b) High-magnification TEM micrograph (c) A schematic representation of the PbSe crystal structure. (d) HRTEM image and its corresponding ED pattern (inset) of a single PbSe nanocube. (e) EDS spectrum taken from a single PbSe nanocrystal.
from a TEM grid) and are free of oxygen and other impurity contaminations. Figure 2a shows a representative TEM image of the PbSe nanoparticles, revealing that the product is made up of a large quantity of nanocubes with a mean edge length ranging from 10 to 40 nm. Figure 2b is a high-magnification TEM image of a single cube which has very smooth faces and sharp edges, with a mean volume size of ∼30 × 30 nm3. Figure 2c schematically illustrates a unit cell of the PbSe structure in which the six surfaces of the PbSe nanocube consist of {200} lattice planes. Figure 2d shows a HRTEM image of a PbSe cube. As seen from the image, the cube is structurally uniform, and no dislocations or other planar defects are observed within it; in fact, the uniformity is perfectly maintained throughout the whole cube. The lattice fringes of the {200} planes with a d spacing
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Figure 4. TEM images showing Zn nanocrystals with various regular geometrical shapes: (a) rectangle, (b) rhombus, (c) triangle, and (d) hexagon.
Figure 3. TEM results of the PbTe nanocubes (a) Low-magnification TEM micrograph. (b) and (c) High-magnification TEM micrograph showing the PbTe nanocrystallites that display a shape of a platform and a rectangular parallelepiped, respectively. (d) EDS spectrum obtained from a single PbTe nanocrystal.
of 0.306 nm are clearly resolved. The corresponding ED pattern (upper right inset), obtained by directing the incident electron beam perpendicular to top facet of the cube, can be indexed as the diffraction in [100] zone axis of the fcc PbSe crystal (JCPDS card no. 06-0354, space group Fm3jm, a ) 6.124 Å). EDS analysis, Figure 2e, shows that the cubes consist of Pb and Se with a Pb/Se atomic ratio of ∼1.0. (Cu signals originate from a TEM grid and are free of oxygen and other impurity contaminations.) Figure 3a shows a TEM image of the PbTe nanoparticles. Some of the particles have a shape of cube, similarly to the cases of PbS and PbSe, while some display a shape of platform and rectangular parallelepiped as shown in Figure 3b and 3c. The platform (Figure 3b) has two trapezoid side planes and the edge lengths are not entirely equivalent. The rectangular parallelepiped (Figure 3c) has a short edge length of ∼ 20 nm and a longer edge length of ∼ 38 nm. EDS analysis (Figure 3d) reveals that the cubes are composed of Pb and Te with a Pb/Te atomic ratio of ∼ 1.0. (Cu signals originate from a TEM grid), and free of oxygen and other impurity contaminations. Careful HRTEM image and ED pattern examinations (not shown) indicate that the PbTe nanoparticles are well-structured and defect-free fcc single crystals and each faceted PbTe
Figure 5. TEM micrographs showing liquid Ga nanoparticles at low magnification (a) and high magnification (b) and a selected area ED pattern of the nanoparticles.
nanocube is enclosed by six {100} crystal planes, as suggested in the cases of the same fcc structured PbS and PbSe nanoparticles. The present EBI method is not only efficient in producing IV-VI semiconducting PbE but is also effective in producing nanocrystals of low-melting metals such as Zn, Cd, Sn, Ga, and In that have not yet perfectly synthesized in previous studies.21-24 It is interesting to note that the Zn nanocrystals prepared through the present EBI approach display various regular geometrical shapes such as rectangle (Figure 4a), rhombus (Figure 4b), triangle (Figure 4c), and hexagon (Figure 4d). These nanocrystallites have a very thin thickness (several nanometers) and mean diameter of ∼30-80 nm. Figure 5 shows TEM images of Ga nanoparticles. The particles display a perfect spherical shape with a diameter of about 20-80 nm and uniformly distributed on the carbon film. ED patterns taken from a large number of Ga nanoparticles and from individual Ga nanoparticle (using a convergent beam) both show only diffuse rings, suggesting that the Ga nanoparticles are in liquid state. It is known that the melting point of nanostructured materials can be much lower than that of their bulky counterparts (for example, the difference between the melting point of Au nanoparticles and Au bulk material is over ∼400 °C31,32). In our laboratory, Ga confined in carbon nanotubes was found to exhibit unusual freezing and melting
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Figure 6. TEM resultes of the Sn nanoparticles. (a) A lowmagnification TEM image of the round shaped Sn nanocrystals covering the C film. (b) A HRTEM image of a single Sn nanocrystal, viewed along [001]. (c) The digital Fourier transform of the image shown in part b indexed as the β-Sn diffraction in [001] zone axis. (d) A HRTEM image of another Sn nanocrystal, viewed along [100]. (e) The digital Fourier transform of the image shown in (d) indexed as the β-Sn diffraction in [100] zone axis.
behavior, remaining liquid down to temperatures as low as -80 °C.33 It is reasonable to assume that there is a significant difference between the melting point of the present tiny Ga nanoparticles and a Ga bulky material (bulk Ga: mp of 29.8 °C), and the present Ga nanoparticles may keep its liquid state to even a much lower temperature. Figure 6a shows a TEM image of the Sn nanocrystals prepared by the EBI method. The Sn nanocrystals are usually in a round shape with smooth surfaces and homogenously distributed on the supporting carbon film. The diameter of the nanocrystals can be reasonably tuned from about 10 to 50 nm by altering the current intensity and the brightness of the electron beam. Atomic plane spacing measurements based on the electron diffraction (upper right inset) indicated that the first six diffraction rings can be interpreted as the (200), (101), (220), (211), (301), and (112) reflections of a tetragonal (β-Sn) structure with lattice constants of a ) 0.582 nm and c ) 0.316 nm, which are in agreement with the data in literature (JCPDS card no. 04-0673, space group I41/amd, a ) 5.831 Å, and c ) 3.182 Å). HRTEM images taken from two individual Sn nanocrystals both with a diameter of ∼15 nm are shown in parts b and d of Figure 6. They are both well-structured single crystal and free of (twin and multiply twins) defect. The marked interplanar d spacings of ∼0.29 and 0.279 nm correspond to those of the {200} and {101} planes, respectively. Parts c and e of Figure 6 are the digital Fourier transforms of the images shown in parts b and d of Figure 6, which can be indexed as the diffraction of the β-Sn in [001] and [100] zone axis,, respectively. During the typical EBI process, the electron current density of the convergent beam used is in the range of ∼125-165 pA/ cm2, which can be varied by changing a beam size and its brightness. Generally, the higher current density, the more electron dose, the smaller diameter of the electron beam, the faster the growth of the involved nanocrystals. It is noted that when the nanocrystals (e.g., PbTe) grow reaching a maximum
Hu et al. length of ∼40-50 nm after 2-3 min irradiation the growth of the PbTe nanocrystals will terminate, i.e., there is not any further growth by increasing the duration of electron beam irradiation. So, it is believed that the critical growth length of the PbTe nanocrystals is ∼40-50 nm. It is also found that the dose of electron beam used for the IV-VI semiconductor lead chalcogenides (PbS, PbSe, and PbTe) is higher than those of the low melting point metals (Zn, Ga, Sn, and Cd). However, these detailed effects of the convergent electron beam (such as the critical radius of the convergent electron beam) on the growth of these mentioned nanocrystals are yet fully understood and require more systematic investigations. In the present study, in situ EBI was performed in a fieldemission TEM operating at an accelerating voltage of 200 kV. The electron current density on the specimen was varied by changing the diameter and brightness of the electron beam. As we know, the incident electron energy, Ep, is determined from the equation Ep ) mc2(1/(1 - β2)1/2 - 1), where m is the rest mass of an electron, c is the speed of light, V is the velocity of an electron, and β ) c/V.34 When an electron passes through a specimen, its energy may partially be transferred to thermal energy and thus may result in the local temperature rise or the displacement of atoms. Many different calculations have been performed to describe the temperature rise in TEM specimens caused by an electron beam.35-38 Here, we use an analysis developed by Yokoba39 to evaluate the temperature increment in a given specimen, e.g., a Sn particle. In our case, the Sn particle is on top of a carbon film (rather than embedded in a carbon film), and thus heat loss through the carbon film conducting is limited. The cylindrical thermal spikes were assumed to be generated in the Sn particle by the incident electrons as they passed through the particle, similar to the Yokoba model.38 The temperature increment ∆Te can be expressed as ∆Te ≈ DJQ, where the temperature increment coefficient D is constant for a given material, J is the electron beam current density, and Q is the total energy loss of the electron. It is apparent that the temperature increment increases with increasing the electron beam current density J and the total energy loss of the electron Q. By use of this equation, it is possible to calculate the temperature increment produced for various electron beam energies and current densities and also to determine the current densities required to heat the particle for various incident electron energies. When the source material, Sn particle, is irradiated, and thus heated by the local temperature plume caused by the incident electron beam, it starts to evaporate and redeposit in the form of nanoclusters by aggregation of Sn molecules on the C film. As the EBI proceeds, more Sn vapor is generated and more Sn nanoclusters are formed. The as-formed Sn nanoclusters are energetically favorable for serving as the stable sites for rapid adhesion of additional Sn molecules that result in the formation of the Sn nanocrystallites. The nucleation and growth mechanism of the EBI method is still unclear. But it was found that a carbon film on a TEM mesh is a good substrate for vapor deposition and renucleation inside the TEM chamber at a very low basic pressure (of ∼1 × 10-5). It can be clearly seen from Figure 7 that the favored sites for Sn and Ga nanoparticles’ nucleation are on the holes’ edges of the carbon film, where the surface is relative rough. We did not notice deposition of any source materials on the Cu grid. As shown in Figure 7, close to the grid there is no Sn and Ga deposition on the carbon film. Carbon is a poor conductor of heat compared with Cu grid; thus, close to the Cu grid the local temperature of the carbon film is higher than that of the center area. The favorite sites for nucleation of the Sn and Ga
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J. Phys. Chem. C, Vol. 113, No. 13, 2009 5205 Project (Grant No. B603), and the Program of Introducing Talents of Discipline to Universities (Grant No. 111-2-04). References and Notes
Figure 7. TEM images showing Ga (a) and Sn (b) nanoparticles nucleated at the edges of the holes’ in the C films.
nanoparticles are the holes’ edges in the carbon film; therefore, it is believed that the rough carbon film edges might have assisted nucleation in growing the above nanocrystals. A similar carbon film edge-assisted growth phenomenon was also observed in growing copper nanorods via a vacuum vapor deposition method.37 4. Conclusion In summary, in situ electron beam irradiation in a TEM was performed to fabricate semiconducting and metallic nanocrystals, including IV-VI semiconducting lead chalcogenides PbE (E ) S, Se, and Te) and low-melting-point metals (Zn, Ga, Sn, In, and Cd) supported on a C thin film. These nanocrystals are structurally uniform and free of any linear and planar defect and do not have any other impurity contamination. It was found that a temperature rise caused by electron thermal spikes in the source particles and that the poor thermal conduction of the C film is responsible for the melting of the source particles well below the actual melting temperature. While a C film is a good substrate for nanocrystal nucleation and growth during vapor deposition, it is found that the favorite sites for nanocrystallite deposition and renucleation are the edges of the holes in the carbon film. Though the nucleation and growth mechanism of the nanocrystallite by the present approach is still unclear, it is believed that this new method can also be employed to synthesize many other material nanocrystals and one-dimensional nanostructures, beyond semiconductors and (low-meltingpoint) metals, such as oxides, sulfides, and nitrides. Acknowledgment. This work was supported by the Program for New Century Excellent Talents of the University in China, the National Natural Science Foundation of China (Grant No. 50872020), the “Dawn” Program of Shanghai Education Commission, China (Grant No. 08SG32), the Program for the Specially Appointed Professor by Donghua University (Shanghai, P. R. China), the Shanghai Leading Academic Discipline
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