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Strain study of gold nanomaterials as HR-TEM calibration standard
Micron 79 (2015) 46–52
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Strain study of gold nanomaterials as HR-TEM calibration standard X.Y. Peng a , L.Q. Zhou a , X. Li b,∗ , X.F. Tao b , L.L. Ren b,∗ , W.H. Cao c , G.F. Xu a a
School of Materials Science and Engineering, Central South University, Changsha 410083, China Division of Nano Metrology and Materials Measurement, National Institute of Metrology, Beijing 100029, China c Division of Electricity and Magnetism, National Institute of Metrology, Beijing 100029, China b
a r t i c l e
i n f o
Article history: Received 19 April 2015 Received in revised form 23 July 2015 Accepted 23 July 2015 Available online 26 July 2015 Keywords: High resolution electron microscopy Geometric phase analysis Strain Interplanar spacing Gold thin film
a b s t r a c t This work presents the use of high resolution electron microscopy (HREM) and geometric phase analysis (GPA) to measure the interplanar spacing and strain distribution of three gold nanomaterials, respectively. The results showed that the {1 1 1} strain was smaller than the {0 0 2} strain for any kind of gold materials at the condition of same measuring method. The 0.65% of {1 1 1} strain in gold film measured by HREM (0.26% measured by GPA) was smaller than the {1 1 1} strains in two gold particles. The presence of lattice strain was interpreted according to the growth mechanism of metallic thin film. It is deduced that the {1 1 1} interplanar spacing of the gold thin film is suitable for high magnification calibration of transmission electron microscopy (TEM) and the gold film is potential to be a new calibration standard of TEM. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Transmission electron microscope is one of the most powerful instruments for studying the microstructure of materials. In crystalline materials the atomic location and lattice arrangement can be determined by employing HREM, which is widely used in the research and development of advanced materials (Jia et al., 2008, 2009). Aberration correctors and image reconstruction methods have pushed the point resolution of TEM to about 0.05 nm (Urban, 2008). With the increasing requirement of HREM images, an accurate magnification calibration of TEM becomes more and more critical. Manufacturers of TEM supply magnification values for each image magnification step for their instruments, but these values have been observed to be in error by up to 10% (McCaffrey and Baribeau, 1995). These errors can occur due to variations in lenses of different microscopes as well as through power supply variations and aging electronic components. Therefore, the calibration of magnification, especially high magnification, must be carried out for quantitative TEM work. Single-crystal silicon and gold nanoparticles are usually used as reference materials to calibrate the high magnification of TEM (the microscope magnification exceeds 300,000 times). In this case, the magnification calibration is performed using a known interplanar spacing of single crystals
∗ Corresponding authors. Fax: +86 10 64524963. E-mail addresses: [email protected] (X. Li), [email protected] (L.L. Ren). http://dx.doi.org/10.1016/j.micron.2015.07.009 0968-4328/© 2015 Elsevier Ltd. All rights reserved.
and thus the image scale can be traceable to the International System of Units definition of length through an atomic lattice constant (Danzebrink et al., 2003). However, the preparation of singlecrystal silicon reference material is costly and time-consuming, which need to use ion beam milling. The electron transparent area of single-crystal silicon is extremely fragile and easily broken. Furthermore, the dislocation density in single-crystal silicon is very large and the strain field induced by dislocation will finally lead to the distortion of HREM images (Gao and Kakimoto, 2014; Shehadeh and Cheng, 2006). Oxygen absorption will slowly proceed when single-crystal silicon is exposed to air, which also can cause lattice distortion (Möller et al., 2002; Yonemura et al., 1998). Gold nanoparticles are also used as calibration standard due to its easy preparation and good structural stability. However, the main structural feature of gold nanoparticles is five-fold rotational twinning parallel to [1 1 0] axis. As a result of this structure, real gold nanoparticles are intrinsically strained (Johnson et al., 2008; Howie and Marks, 1984). Moreover, the stability of lacey carbon film with deposited gold nanoparticles is bad due to the charge concentration effect when the magnification and illumination intensity of TEM are high, which leads to the huge difficulty of the acquirement of HREM images. In this work two kinds of gold nanoparticles and a series of polycrystalline gold thin films were prepared and used to develop a new candidate of TEM reference material. The interplanar spacing and strain distribution of three gold nanomaterials were measured by employing HREM and GPA, respectively. The causes of lattice strain
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in gold nanomaterials were analyzed, which was rarely done when some other TEM reference materials were produced (Jeroen et al., 2006; Derose and Revel, 1999). 2. Experimental Gold nanoparticles were synthesized by chemical reduction method within the suspension of chloroauric acid and sodium citrate. All glasswares were firstly rinsed thoroughly by newly prepared aqua regia and then were washed by ultra-pure water. A round-bottom flask was used to mix the 0.5 ml of 1% chloroauric acid solution and 50 ml redistilled water. The solution was then stirred and heated up to 100 ◦ C. 4 ml of 1% trisodium citrate solution was added to the boiling solution, which changed the color of the solution from pale yellow to deep red. The mixed solution was then stirred and cooled to room temperature. The obtained solution was decanted through a membrane with thickness of 0.45 m and then was dispersed by ultrasonic. Lastly, the well-dispersed solution was dipped onto lacey carbon film 300 mesh copper grids for TEM observation. Another kind of gold particles was prepared by the same method, but using 1 ml of 1% trisodium citrate solution instead of 4 ml. Two kinds of gold particles are respectively named as I and II. Polycrystalline gold thin films were deposited onto lacey carbon film of 300 mesh copper TEM grids by Leica EM SCD500 high-vacuum ion-beam sputter. High purity argon and a 99.99% pure Au foil were employed as sputtering gas and target, respectively. The chamber was firstly pumped to a base pressure of below 10−5 mBar and then was increased to 8 × 10−3 mBar for sputtering deposition. A series of polycrystalline gold thin films were obtained at different sputtering parameters of distance, time and current. The crystal phases and lattice parameters of the gold materials were analyzed by X-ray diffraction (D/max-2500 PC, Rigaku) with Cu K␣ radiation ( = 1.54051 Å) at a scan rate of 0.02◦ /step. A Zeiss Libra 200FE TEM operating at 200 kV was used for the observation of gold particles and films. Strain distribution was obtained from the analysis of HREM image using GPA Phase software (HREM Research Inc.), a plug-in for the image processing package Digital Micrograph. The {1 1 1} and {0 0 2} interplanar spacings of gold materials were analyzed using a Fourier-space mask of 2.15 nm−1 which was necessary to reduce noise but limited the spatial resolution of strain measurements to 0.5 nm. 3. Results and discussion The XRD patterns of gold particles and films with different preparation parameters are shown in Fig. 1, from which it is seen that the main diffraction crystallographic planes are Au(1 1 1), Cu(1 1 1) and Cu(2 0 0) (the Cu diffraction peaks are produced by copper grids). The (1 1 1) peak of the I gold particles is higher than that of the II gold particles and the (1 1 0) intensity of gold films increases remarkably with the increase of deposition current, which indicates that the crystallinity is gradually improved in the gold particles and films. The measured (1 1 1) and (2 0 0) interplanar spacings are listed in Table 1, which are very close to the theoretical values. Therefore, these values will be used to calibrate the HREM and GPA measured interplanar spacings in the following sections. Fig. 2 shows the microstructure and strain distribution of the I gold nanoparticles. Prior to HREM test, the magnification of the microscope is calibrated using a MAG*I*CAL® reference material that has perfect lattice and reliable traceability (McCaffrey and Baribeau, 1995), which is treated as the initial state of the microscope. The bright-field (BF) electron micrograph of the I gold nanoparticles shows that finely dispersed particles have diameters of 5–10 nm. Some particles with poor crystallinity are also observed (Fig. 2b). The smallest particles are less suitable for HREM obser-
Fig. 1. XRD patterns of gold particles and films with different preparation parameters.
vation while the larger particles are highly suitable for stronger diffraction. HREM image and corresponding Fast Fourier Transform (FFT) spots of a gold particle with relatively good crystallinity are used to determine its crystal orientations, as shown in Fig. 2b ¯ and (0 0 2) are and c. The measured interplanar spacings of (1 1¯ 1) respectively 0.24950 and 0.22965 nm, which are larger than the XRD measured values of 0.23563 and 0.20377 nm. It is deduced ¯ and [0 0 2] directions of that there are tensile strains in the [1 1¯ 1] the gold particle. The strain maps of the gold particle in the two directions are calculated by GPA, as shown in Fig. 2d and e, respectively. During the calculation of strain, an area with small lattice distortion in the HREM image of the gold particle was chosen as a reference area. This area is recognized in the phase image according to uniform contrast or gradient. With such maps, the strain values can be extracted in different ways, either as area or line or point. The mean strain values of the gold particle, which are calculated from a white square (highlighted in Fig. 2d and e), are respectively ¯ and [0 0 2] directions. 0.66% and 0.95% in the [1 1¯ 1] As shown in Fig. 3a, the BF image of the II gold particles displays that the particles have a size distribution of 10–20 nm and some large particles join together. Some non-spherical gold particles, especially decahedral particles which contain multiply twins (the inset), are also observed. The decahedral gold particles are intrinsically strained and thus the obtained HREM image is distorted. Fig. 3b shows the HREM image and corresponding FFT spots of a gold nanoparticle with relatively good crystallinity. The measured ¯ and (0 0 2) are respectively 0.24741 interplanar spacings of (1 1¯ 1) and 0.22361 nm, which are larger than the XRD measured values (0.23575 and 0.20351 nm). It is indicated that tensile strain exists ¯ and [0 0 2] directions of the gold nanoparticle. Fig. 3c in the [1 1¯ 1] and d present the strain maps of the gold particle in the above two directions. The calculated mean strain values of the gold particle in the two directions are 0.59% and 0.67%, respectively. A deep understanding of the strain states of gold nanoparticles can be obtained with the aid of its simple solid-geometry model. The decahedral particle is composed of five tetrahedral subunits with the face-centred-cubic (fcc) crystal structure and {1 1 1} planes (Fig. 4). The subunits are joined to adjacent tetrahedra by twin boundaries and share one edge with the five-fold axis. However, the tetrahedra do not completely fill space and a solid-angle deficiency remains (Fig. 4b). As a result of this deficiency, the decahedral particle must be strained and/or contains defects. In gold nanoparticles with diameter smaller than 10 nm, the decahedral structure occurs but is unstable. In much larger particles, how-
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Table 1 Lattice parameters of gold materials measured by XRD. Measured d
I
II
d(1 1 1) (nm) d(2 0 0) (nm)
0.23563 0.20377
0.23575 0.20351
Gold thin films (mm, s, mA) 55, 10, 20
37, 10, 20
37, 30, 20
37, 30, 30
0.23587 0.20360
0.23540 0.20394
0.23492 0.20377
0.23516 0.20368
Fig. 2. TEM images and strain maps of I gold nanoparticles: (a) BF image; (b) HREM image and FFT spots; (c) HREM image of a gold particle with calculated interplanar ¯ and [0 0 2]. White square is used for calculating mean strain value. spacings; (d), (e) Strain maps of the particle respectively in [1 1¯ 1]
Fig. 3. TEM images and strain maps of II gold nanoparticles: (a) BF and HREM images; (b) HREM image of a gold particle with calculated interplanar spacings and FFT spots; ¯ and [0 0 2]. (c), (d) Strain maps of the particle respectively in [1 1¯ 1]
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Table 2 Strain values of gold materials measured by HREM and GPA. Measured strains I gold nanoparticles II gold nanoparticles A gold thin film
1 1 1byHREM (%) 1 1 1byGPA (%) 0 0 2byHREM (%) 0 0 2byGPA (%)
Fig. 4. Decahedral geometry of gold nanoparticle: (a) The shaded part is one of five tetrahedral subunits, which have top and bottom {1 1 1} planes (light grey triangles) and are arranged about the five-fold-rotation axis parallel to [1 1 0], the dark grey triangle is the internal twin interface that joins adjacent tetrahedrons. (b) For perfect fcc tetrahedral subunits, the angle between adjacent {1 1 1} faces is 70.53◦ , which results in a 7.35◦ solid-angle deficiency.
ever, dislocations and stacking faults commonly occur (Hofmeister, 1998; Dubiel et al., 1997). Two models for the internal structure of decahedral particles have been debated. Some researchers proposed a homogeneous-strain model, that is, the solid-angle deficiency could be accommodated by a structural transformation from fcc to orthorhombic structure (Yang, 1979). Others proposed a disclination model according to the inhomogeneous strain distributions obtained by electron microscope (Marks et al., 1979; Ino and Ogawa, 1968). In the disclination model the solid-angle deficiency was removed by a single wedge disclination coinciding with the five-fold axis, which produced an inhomogeneous strain field and significantly reduced the strain-energy density of the particles (Gryaznov et al., 1999). Although massive experimental works are in favor of the inhomogeneous-strain model, quantitative strain measurements are rarely obtained. A full analysis of the strain model for the decahedral particles is beyond the scope of this work. The microstructures of gold thin films are determined by the preparation parameters (distance, time and current) used in the process of ion sputtering deposition. Fig. 5 shows the TEM images of gold thin films obtained at different sputtering parameters. When the sputtering distance, time and current are respectively 55 mm, 10 s and 20 mA, the obtained gold film contains abundant islandlike crystallites with diameters of 4–10 nm. The selected-area diffraction (SAD) patterns indicate that these crystallites have different crystallographic orientations (the inset of Fig. 5a). When the sputtering distance is decreased to 37 mm (the other two parameters remain the same), a reticular structure is produced in the gold film (Fig. 5b). The sputtering time is then increased to 30 s; the reticular film develops into continuous film with grain diameters of several nanometers to tens of nanometers. Lastly, the sputtering current is increased to 30 mA, the grain size of the obtained continuous film significantly increases. Some grain boundary dislocations are also observed (the insets of Fig. 5d). As a result, the growth of grains in gold film was promoted by the decrease of sputtering distance, the increase of sputtering time and current. A deposited gold film (37 mm, 30 s, 20 mA), which has intermediate grain size and appropriate thickness for HREM imaging, is chosen to study the strain state of the gold film. The microstructure and strain distribution of the gold film are displayed in Fig. 6. Many nanocrystalline grains and twins with diameters of several nanometers to tens of nanometers are produced in the film (Fig. 6a). The HREM image and corresponding FFT spots of a gold grain with relatively good crystallinity are used to measure crystallographic orientation, interplanar spacing and strain distribution. ¯ and (1 1¯ 1) ¯ are respecThe measured interplanar spacings of (0 0 2) tively 0.17639 and 0.23644 nm, which are different with the XRD measured values of 0.20377 and 0.23492 nm. It is suggested that ¯ and [1 1¯ 1] ¯ direccompressive and tensile strains exist in the [0 0 2] tions of the gold film, respectively. The strain distributions of the
5.89 0.66 12.70 0.95
4.95 0.59 9.88 0.67
0.65 0.26 −13.44 −2.32
gold grain mapped by GPA in the two directions are shown in Fig. 6c and d. The calculated mean strain values of the gold grain are respectively −2.32% and 0.26% in the two directions. The strain values of three gold materials measured by HREM and GPA are listed in Table 2, and the HREM-based strain values are calculated according to the XRD-measured lattice parameters. For any kind of the gold materials, the measured lattice strain of {0 0 2} is larger than that of {1 1 1} at the condition of same measuring method, especially that of {0 0 2} in the gold film reaches to −13.44%. The 0.65% of {1 1 1} strain measured by HREM in the gold film (0.26% measured by GPA) is smaller than the {1 1 1} strains in two gold nanoparticles. In addition, both {1 1 1} and {0 0 2} strains of the gold nanoparticles decrease with the increase of particle size. It is concluded that the {1 1 1} interplanar spacing of the gold film is suitable for high magnification calibration of TEM and the gold thin film is potential to be a new calibration standard of TEM. Moreover, the strain of {1 1 1} or {0 0 2} measured by GPA is smaller than the strain measured by HREM for any kind of gold material, which is related to the chosen reference area of strain calculation. The reference area was chosen from the HREM image which contains lattice strain, therefore, the strained reference area used in GPA led to relatively smaller values of lattice strain. In addition, it can be seen that the strain values of the HREM-observed gold nanoparticles are positive even though these particles do not contain twins and lattice defects. We believe that the main reason for the lattice expansions is electron beam irradiation effect. According to the theoretical calculations (Mohapatra et al., 2013), the electronbeam irradiation leads to temperature rise (about 140 ◦ C) of the observed samples and knock-on displacements of C and O atoms from the organic membrane. The accumulation of heat energy and the diffusion of C and O atoms in the gold nanoparticles result in the lattice expansions. HREM technology is widely used to measure the interplanar spacing of crystal materials, to determine the crystal symmetry, growth orientations and the existence of defects. However, the geometry of HREM image and SAD spot are influenced by the refraction effect of incident beam which produced by surface morphology, misoriented grains, twins, additional foreign phases and extended defects (Cimalla et al., 2002). Fig. 7a–c give the schematic presentation of refraction effect. When an electron beam enters the surface or interface of crystal materials (spherical surface, grain boundary and twin boundary, etc.) it experiences an abrupt potential change. An electron beam has to fulfill the boundary conditions by changing the angle, i.e., it will be refracted at the interface. This will cause the shift of Bragg angle and the resulting spots are usually much broader and elongated compared to pure transmission spots. As shown in Fig. 7d–i, the SAD spots of gold particles and films are small and circular, which indicates that the refraction effect in the gold particles and films was pretty weak. The main reasons are that the surface curvature and thickness of the gold materials are very small and the gold materials were not tilted during HREM observation. Based on the microstructure evolution of the gold films, it was deduced that the gold films grew by Volmer–Weber mechanism (Seel et al., 2000; Fillon et al., 2010), with processes of island nucleation, growth and coalescence. The stain/stress evolution process of metallic thin films (Au, Ag, Al and Cu) could be divided into three
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Fig. 5. TEM images of gold films obtained at different preparation parameters: (a) 55 mm, 10 s, 20 mA, the SAD patterns of the circled region are showed in inset; (b) 37 mm, 10 s, 20 mA; (c) 37 mm, 30 s, 20 mA; (d) 37 mm, 30 s, 30 mA, the insets are the enlarged views of grain boundary dislocations.
Fig. 6. TEM images and strain maps of a gold film: (a) BF image; (b) HREM image of a gold grain with calculated interplanar spacings and FFT spots; (c), (d) Strain maps of ¯ and [1 1¯ 1]. ¯ the gold grain respectively in [0 0 2]
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Fig. 7. Schematic presentation of the refraction effect on: (a) spherical surface; (b) grain boundary; (c) twin boundary; and SAD patterns of gold particles and films: (d) I particle; (e) II particle; (f) 55 mm, 10 s, 20 mA; (g) 37 mm, 10 s, 20 mA; (h) 37 mm, 30 s, 20 mA; (i) 37 mm, 30 s, 30 mA.
stages (Jerrold et al., 2002; Yu and Thompson, 2014). Firstly, the film is in the form of isolated islands and the average film stress is slightly compressive. Secondly, a tensile stress increases rapidly and reaches a maximum value when the film is 100% areal coverage. In the third stage, the tensile stress relaxes and eventually becomes compressive with continued deposition. Therefore, the steady-state stress depends on the film structure at which growth is interrupted. The mechanism behind the increase in tensile stress during coalescence (stage two) was successfully explained by earlier studies (Nix and Clemens, 1999; Freund and Chason, 2001). The main idea is that when two islands contact each other, their total free energy is lowered by forming a grain boundary. They pay an elastic energy penalty in the process of lowering the surface area, leading to a tensile stress. Previous researches also presented a model for compressive stress generation during thin film growth (Chason et al., 2002; Guduru et al., 2003). During deposition, the free surface is at a non-equilibrium state with a supersaturated population of adatoms and possesses an excess chemical potential being greater than zero. When island coalescence is complete, the grain boundaries are under a tensile stress and hence its chemical potential is negative. The excess chemical potential on the surface drives adatoms into the grain boundaries. Extra atoms are inserted into the grain boundaries leads to crystal defects (dislocations) and a transformation from tensile stress to compressive stress. The large compressive {0 0 2} strain and the grain boundary dislocations in gold films can be interpreted according to the above growth mechanism of metallic thin film. Further works are performed to study the relationship between the {0 0 2} strain and the grain boundary dislocations and to evaluate the measurement repeatability and accuracy of {1 1 1} interplanar spacing in the gold film. 4. Conclusions This work employed HREM and GPA to investigate the strain states of three gold materials and thus to develop a new calibration standard of TEM. It was revealed that the lattice strain of {1 1 1} was
smaller than that of {0 0 2} for any kind of gold material at the condition of same measuring method. The 0.65% of {1 1 1} strain in the gold film measured by HREM (0.26% measured by GPA) was smaller than the {1 1 1} strain of two gold nanoparticles. It is deduced that the {1 1 1} interplanar spacing of the gold film was suitable for high magnification calibration of TEM and the gold film was potential to be a new calibration standard of TEM. Moreover, the large compressive strain of {0 0 2} in the gold film was interpreted according to the growth mechanism of metallic thin film.
Acknowledgments This work was supported by the National Key Technology R&D Program with No. 2011BAK15B00.
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Micron journal homepage: www.elsevier.com/locate/micron
Strain study of gold nanomaterials as HR-TEM calibration standard X.Y. Peng a , L.Q. Zhou a , X. Li b,∗ , X.F. Tao b , L.L. Ren b,∗ , W.H. Cao c , G.F. Xu a a
School of Materials Science and Engineering, Central South University, Changsha 410083, China Division of Nano Metrology and Materials Measurement, National Institute of Metrology, Beijing 100029, China c Division of Electricity and Magnetism, National Institute of Metrology, Beijing 100029, China b
a r t i c l e
i n f o
Article history: Received 19 April 2015 Received in revised form 23 July 2015 Accepted 23 July 2015 Available online 26 July 2015 Keywords: High resolution electron microscopy Geometric phase analysis Strain Interplanar spacing Gold thin film
a b s t r a c t This work presents the use of high resolution electron microscopy (HREM) and geometric phase analysis (GPA) to measure the interplanar spacing and strain distribution of three gold nanomaterials, respectively. The results showed that the {1 1 1} strain was smaller than the {0 0 2} strain for any kind of gold materials at the condition of same measuring method. The 0.65% of {1 1 1} strain in gold film measured by HREM (0.26% measured by GPA) was smaller than the {1 1 1} strains in two gold particles. The presence of lattice strain was interpreted according to the growth mechanism of metallic thin film. It is deduced that the {1 1 1} interplanar spacing of the gold thin film is suitable for high magnification calibration of transmission electron microscopy (TEM) and the gold film is potential to be a new calibration standard of TEM. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Transmission electron microscope is one of the most powerful instruments for studying the microstructure of materials. In crystalline materials the atomic location and lattice arrangement can be determined by employing HREM, which is widely used in the research and development of advanced materials (Jia et al., 2008, 2009). Aberration correctors and image reconstruction methods have pushed the point resolution of TEM to about 0.05 nm (Urban, 2008). With the increasing requirement of HREM images, an accurate magnification calibration of TEM becomes more and more critical. Manufacturers of TEM supply magnification values for each image magnification step for their instruments, but these values have been observed to be in error by up to 10% (McCaffrey and Baribeau, 1995). These errors can occur due to variations in lenses of different microscopes as well as through power supply variations and aging electronic components. Therefore, the calibration of magnification, especially high magnification, must be carried out for quantitative TEM work. Single-crystal silicon and gold nanoparticles are usually used as reference materials to calibrate the high magnification of TEM (the microscope magnification exceeds 300,000 times). In this case, the magnification calibration is performed using a known interplanar spacing of single crystals
∗ Corresponding authors. Fax: +86 10 64524963. E-mail addresses: [email protected] (X. Li), [email protected] (L.L. Ren). http://dx.doi.org/10.1016/j.micron.2015.07.009 0968-4328/© 2015 Elsevier Ltd. All rights reserved.
and thus the image scale can be traceable to the International System of Units definition of length through an atomic lattice constant (Danzebrink et al., 2003). However, the preparation of singlecrystal silicon reference material is costly and time-consuming, which need to use ion beam milling. The electron transparent area of single-crystal silicon is extremely fragile and easily broken. Furthermore, the dislocation density in single-crystal silicon is very large and the strain field induced by dislocation will finally lead to the distortion of HREM images (Gao and Kakimoto, 2014; Shehadeh and Cheng, 2006). Oxygen absorption will slowly proceed when single-crystal silicon is exposed to air, which also can cause lattice distortion (Möller et al., 2002; Yonemura et al., 1998). Gold nanoparticles are also used as calibration standard due to its easy preparation and good structural stability. However, the main structural feature of gold nanoparticles is five-fold rotational twinning parallel to [1 1 0] axis. As a result of this structure, real gold nanoparticles are intrinsically strained (Johnson et al., 2008; Howie and Marks, 1984). Moreover, the stability of lacey carbon film with deposited gold nanoparticles is bad due to the charge concentration effect when the magnification and illumination intensity of TEM are high, which leads to the huge difficulty of the acquirement of HREM images. In this work two kinds of gold nanoparticles and a series of polycrystalline gold thin films were prepared and used to develop a new candidate of TEM reference material. The interplanar spacing and strain distribution of three gold nanomaterials were measured by employing HREM and GPA, respectively. The causes of lattice strain
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in gold nanomaterials were analyzed, which was rarely done when some other TEM reference materials were produced (Jeroen et al., 2006; Derose and Revel, 1999). 2. Experimental Gold nanoparticles were synthesized by chemical reduction method within the suspension of chloroauric acid and sodium citrate. All glasswares were firstly rinsed thoroughly by newly prepared aqua regia and then were washed by ultra-pure water. A round-bottom flask was used to mix the 0.5 ml of 1% chloroauric acid solution and 50 ml redistilled water. The solution was then stirred and heated up to 100 ◦ C. 4 ml of 1% trisodium citrate solution was added to the boiling solution, which changed the color of the solution from pale yellow to deep red. The mixed solution was then stirred and cooled to room temperature. The obtained solution was decanted through a membrane with thickness of 0.45 m and then was dispersed by ultrasonic. Lastly, the well-dispersed solution was dipped onto lacey carbon film 300 mesh copper grids for TEM observation. Another kind of gold particles was prepared by the same method, but using 1 ml of 1% trisodium citrate solution instead of 4 ml. Two kinds of gold particles are respectively named as I and II. Polycrystalline gold thin films were deposited onto lacey carbon film of 300 mesh copper TEM grids by Leica EM SCD500 high-vacuum ion-beam sputter. High purity argon and a 99.99% pure Au foil were employed as sputtering gas and target, respectively. The chamber was firstly pumped to a base pressure of below 10−5 mBar and then was increased to 8 × 10−3 mBar for sputtering deposition. A series of polycrystalline gold thin films were obtained at different sputtering parameters of distance, time and current. The crystal phases and lattice parameters of the gold materials were analyzed by X-ray diffraction (D/max-2500 PC, Rigaku) with Cu K␣ radiation ( = 1.54051 Å) at a scan rate of 0.02◦ /step. A Zeiss Libra 200FE TEM operating at 200 kV was used for the observation of gold particles and films. Strain distribution was obtained from the analysis of HREM image using GPA Phase software (HREM Research Inc.), a plug-in for the image processing package Digital Micrograph. The {1 1 1} and {0 0 2} interplanar spacings of gold materials were analyzed using a Fourier-space mask of 2.15 nm−1 which was necessary to reduce noise but limited the spatial resolution of strain measurements to 0.5 nm. 3. Results and discussion The XRD patterns of gold particles and films with different preparation parameters are shown in Fig. 1, from which it is seen that the main diffraction crystallographic planes are Au(1 1 1), Cu(1 1 1) and Cu(2 0 0) (the Cu diffraction peaks are produced by copper grids). The (1 1 1) peak of the I gold particles is higher than that of the II gold particles and the (1 1 0) intensity of gold films increases remarkably with the increase of deposition current, which indicates that the crystallinity is gradually improved in the gold particles and films. The measured (1 1 1) and (2 0 0) interplanar spacings are listed in Table 1, which are very close to the theoretical values. Therefore, these values will be used to calibrate the HREM and GPA measured interplanar spacings in the following sections. Fig. 2 shows the microstructure and strain distribution of the I gold nanoparticles. Prior to HREM test, the magnification of the microscope is calibrated using a MAG*I*CAL® reference material that has perfect lattice and reliable traceability (McCaffrey and Baribeau, 1995), which is treated as the initial state of the microscope. The bright-field (BF) electron micrograph of the I gold nanoparticles shows that finely dispersed particles have diameters of 5–10 nm. Some particles with poor crystallinity are also observed (Fig. 2b). The smallest particles are less suitable for HREM obser-
Fig. 1. XRD patterns of gold particles and films with different preparation parameters.
vation while the larger particles are highly suitable for stronger diffraction. HREM image and corresponding Fast Fourier Transform (FFT) spots of a gold particle with relatively good crystallinity are used to determine its crystal orientations, as shown in Fig. 2b ¯ and (0 0 2) are and c. The measured interplanar spacings of (1 1¯ 1) respectively 0.24950 and 0.22965 nm, which are larger than the XRD measured values of 0.23563 and 0.20377 nm. It is deduced ¯ and [0 0 2] directions of that there are tensile strains in the [1 1¯ 1] the gold particle. The strain maps of the gold particle in the two directions are calculated by GPA, as shown in Fig. 2d and e, respectively. During the calculation of strain, an area with small lattice distortion in the HREM image of the gold particle was chosen as a reference area. This area is recognized in the phase image according to uniform contrast or gradient. With such maps, the strain values can be extracted in different ways, either as area or line or point. The mean strain values of the gold particle, which are calculated from a white square (highlighted in Fig. 2d and e), are respectively ¯ and [0 0 2] directions. 0.66% and 0.95% in the [1 1¯ 1] As shown in Fig. 3a, the BF image of the II gold particles displays that the particles have a size distribution of 10–20 nm and some large particles join together. Some non-spherical gold particles, especially decahedral particles which contain multiply twins (the inset), are also observed. The decahedral gold particles are intrinsically strained and thus the obtained HREM image is distorted. Fig. 3b shows the HREM image and corresponding FFT spots of a gold nanoparticle with relatively good crystallinity. The measured ¯ and (0 0 2) are respectively 0.24741 interplanar spacings of (1 1¯ 1) and 0.22361 nm, which are larger than the XRD measured values (0.23575 and 0.20351 nm). It is indicated that tensile strain exists ¯ and [0 0 2] directions of the gold nanoparticle. Fig. 3c in the [1 1¯ 1] and d present the strain maps of the gold particle in the above two directions. The calculated mean strain values of the gold particle in the two directions are 0.59% and 0.67%, respectively. A deep understanding of the strain states of gold nanoparticles can be obtained with the aid of its simple solid-geometry model. The decahedral particle is composed of five tetrahedral subunits with the face-centred-cubic (fcc) crystal structure and {1 1 1} planes (Fig. 4). The subunits are joined to adjacent tetrahedra by twin boundaries and share one edge with the five-fold axis. However, the tetrahedra do not completely fill space and a solid-angle deficiency remains (Fig. 4b). As a result of this deficiency, the decahedral particle must be strained and/or contains defects. In gold nanoparticles with diameter smaller than 10 nm, the decahedral structure occurs but is unstable. In much larger particles, how-
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Table 1 Lattice parameters of gold materials measured by XRD. Measured d
I
II
d(1 1 1) (nm) d(2 0 0) (nm)
0.23563 0.20377
0.23575 0.20351
Gold thin films (mm, s, mA) 55, 10, 20
37, 10, 20
37, 30, 20
37, 30, 30
0.23587 0.20360
0.23540 0.20394
0.23492 0.20377
0.23516 0.20368
Fig. 2. TEM images and strain maps of I gold nanoparticles: (a) BF image; (b) HREM image and FFT spots; (c) HREM image of a gold particle with calculated interplanar ¯ and [0 0 2]. White square is used for calculating mean strain value. spacings; (d), (e) Strain maps of the particle respectively in [1 1¯ 1]
Fig. 3. TEM images and strain maps of II gold nanoparticles: (a) BF and HREM images; (b) HREM image of a gold particle with calculated interplanar spacings and FFT spots; ¯ and [0 0 2]. (c), (d) Strain maps of the particle respectively in [1 1¯ 1]
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Table 2 Strain values of gold materials measured by HREM and GPA. Measured strains I gold nanoparticles II gold nanoparticles A gold thin film
1 1 1byHREM (%) 1 1 1byGPA (%) 0 0 2byHREM (%) 0 0 2byGPA (%)
Fig. 4. Decahedral geometry of gold nanoparticle: (a) The shaded part is one of five tetrahedral subunits, which have top and bottom {1 1 1} planes (light grey triangles) and are arranged about the five-fold-rotation axis parallel to [1 1 0], the dark grey triangle is the internal twin interface that joins adjacent tetrahedrons. (b) For perfect fcc tetrahedral subunits, the angle between adjacent {1 1 1} faces is 70.53◦ , which results in a 7.35◦ solid-angle deficiency.
ever, dislocations and stacking faults commonly occur (Hofmeister, 1998; Dubiel et al., 1997). Two models for the internal structure of decahedral particles have been debated. Some researchers proposed a homogeneous-strain model, that is, the solid-angle deficiency could be accommodated by a structural transformation from fcc to orthorhombic structure (Yang, 1979). Others proposed a disclination model according to the inhomogeneous strain distributions obtained by electron microscope (Marks et al., 1979; Ino and Ogawa, 1968). In the disclination model the solid-angle deficiency was removed by a single wedge disclination coinciding with the five-fold axis, which produced an inhomogeneous strain field and significantly reduced the strain-energy density of the particles (Gryaznov et al., 1999). Although massive experimental works are in favor of the inhomogeneous-strain model, quantitative strain measurements are rarely obtained. A full analysis of the strain model for the decahedral particles is beyond the scope of this work. The microstructures of gold thin films are determined by the preparation parameters (distance, time and current) used in the process of ion sputtering deposition. Fig. 5 shows the TEM images of gold thin films obtained at different sputtering parameters. When the sputtering distance, time and current are respectively 55 mm, 10 s and 20 mA, the obtained gold film contains abundant islandlike crystallites with diameters of 4–10 nm. The selected-area diffraction (SAD) patterns indicate that these crystallites have different crystallographic orientations (the inset of Fig. 5a). When the sputtering distance is decreased to 37 mm (the other two parameters remain the same), a reticular structure is produced in the gold film (Fig. 5b). The sputtering time is then increased to 30 s; the reticular film develops into continuous film with grain diameters of several nanometers to tens of nanometers. Lastly, the sputtering current is increased to 30 mA, the grain size of the obtained continuous film significantly increases. Some grain boundary dislocations are also observed (the insets of Fig. 5d). As a result, the growth of grains in gold film was promoted by the decrease of sputtering distance, the increase of sputtering time and current. A deposited gold film (37 mm, 30 s, 20 mA), which has intermediate grain size and appropriate thickness for HREM imaging, is chosen to study the strain state of the gold film. The microstructure and strain distribution of the gold film are displayed in Fig. 6. Many nanocrystalline grains and twins with diameters of several nanometers to tens of nanometers are produced in the film (Fig. 6a). The HREM image and corresponding FFT spots of a gold grain with relatively good crystallinity are used to measure crystallographic orientation, interplanar spacing and strain distribution. ¯ and (1 1¯ 1) ¯ are respecThe measured interplanar spacings of (0 0 2) tively 0.17639 and 0.23644 nm, which are different with the XRD measured values of 0.20377 and 0.23492 nm. It is suggested that ¯ and [1 1¯ 1] ¯ direccompressive and tensile strains exist in the [0 0 2] tions of the gold film, respectively. The strain distributions of the
5.89 0.66 12.70 0.95
4.95 0.59 9.88 0.67
0.65 0.26 −13.44 −2.32
gold grain mapped by GPA in the two directions are shown in Fig. 6c and d. The calculated mean strain values of the gold grain are respectively −2.32% and 0.26% in the two directions. The strain values of three gold materials measured by HREM and GPA are listed in Table 2, and the HREM-based strain values are calculated according to the XRD-measured lattice parameters. For any kind of the gold materials, the measured lattice strain of {0 0 2} is larger than that of {1 1 1} at the condition of same measuring method, especially that of {0 0 2} in the gold film reaches to −13.44%. The 0.65% of {1 1 1} strain measured by HREM in the gold film (0.26% measured by GPA) is smaller than the {1 1 1} strains in two gold nanoparticles. In addition, both {1 1 1} and {0 0 2} strains of the gold nanoparticles decrease with the increase of particle size. It is concluded that the {1 1 1} interplanar spacing of the gold film is suitable for high magnification calibration of TEM and the gold thin film is potential to be a new calibration standard of TEM. Moreover, the strain of {1 1 1} or {0 0 2} measured by GPA is smaller than the strain measured by HREM for any kind of gold material, which is related to the chosen reference area of strain calculation. The reference area was chosen from the HREM image which contains lattice strain, therefore, the strained reference area used in GPA led to relatively smaller values of lattice strain. In addition, it can be seen that the strain values of the HREM-observed gold nanoparticles are positive even though these particles do not contain twins and lattice defects. We believe that the main reason for the lattice expansions is electron beam irradiation effect. According to the theoretical calculations (Mohapatra et al., 2013), the electronbeam irradiation leads to temperature rise (about 140 ◦ C) of the observed samples and knock-on displacements of C and O atoms from the organic membrane. The accumulation of heat energy and the diffusion of C and O atoms in the gold nanoparticles result in the lattice expansions. HREM technology is widely used to measure the interplanar spacing of crystal materials, to determine the crystal symmetry, growth orientations and the existence of defects. However, the geometry of HREM image and SAD spot are influenced by the refraction effect of incident beam which produced by surface morphology, misoriented grains, twins, additional foreign phases and extended defects (Cimalla et al., 2002). Fig. 7a–c give the schematic presentation of refraction effect. When an electron beam enters the surface or interface of crystal materials (spherical surface, grain boundary and twin boundary, etc.) it experiences an abrupt potential change. An electron beam has to fulfill the boundary conditions by changing the angle, i.e., it will be refracted at the interface. This will cause the shift of Bragg angle and the resulting spots are usually much broader and elongated compared to pure transmission spots. As shown in Fig. 7d–i, the SAD spots of gold particles and films are small and circular, which indicates that the refraction effect in the gold particles and films was pretty weak. The main reasons are that the surface curvature and thickness of the gold materials are very small and the gold materials were not tilted during HREM observation. Based on the microstructure evolution of the gold films, it was deduced that the gold films grew by Volmer–Weber mechanism (Seel et al., 2000; Fillon et al., 2010), with processes of island nucleation, growth and coalescence. The stain/stress evolution process of metallic thin films (Au, Ag, Al and Cu) could be divided into three
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Fig. 5. TEM images of gold films obtained at different preparation parameters: (a) 55 mm, 10 s, 20 mA, the SAD patterns of the circled region are showed in inset; (b) 37 mm, 10 s, 20 mA; (c) 37 mm, 30 s, 20 mA; (d) 37 mm, 30 s, 30 mA, the insets are the enlarged views of grain boundary dislocations.
Fig. 6. TEM images and strain maps of a gold film: (a) BF image; (b) HREM image of a gold grain with calculated interplanar spacings and FFT spots; (c), (d) Strain maps of ¯ and [1 1¯ 1]. ¯ the gold grain respectively in [0 0 2]
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Fig. 7. Schematic presentation of the refraction effect on: (a) spherical surface; (b) grain boundary; (c) twin boundary; and SAD patterns of gold particles and films: (d) I particle; (e) II particle; (f) 55 mm, 10 s, 20 mA; (g) 37 mm, 10 s, 20 mA; (h) 37 mm, 30 s, 20 mA; (i) 37 mm, 30 s, 30 mA.
stages (Jerrold et al., 2002; Yu and Thompson, 2014). Firstly, the film is in the form of isolated islands and the average film stress is slightly compressive. Secondly, a tensile stress increases rapidly and reaches a maximum value when the film is 100% areal coverage. In the third stage, the tensile stress relaxes and eventually becomes compressive with continued deposition. Therefore, the steady-state stress depends on the film structure at which growth is interrupted. The mechanism behind the increase in tensile stress during coalescence (stage two) was successfully explained by earlier studies (Nix and Clemens, 1999; Freund and Chason, 2001). The main idea is that when two islands contact each other, their total free energy is lowered by forming a grain boundary. They pay an elastic energy penalty in the process of lowering the surface area, leading to a tensile stress. Previous researches also presented a model for compressive stress generation during thin film growth (Chason et al., 2002; Guduru et al., 2003). During deposition, the free surface is at a non-equilibrium state with a supersaturated population of adatoms and possesses an excess chemical potential being greater than zero. When island coalescence is complete, the grain boundaries are under a tensile stress and hence its chemical potential is negative. The excess chemical potential on the surface drives adatoms into the grain boundaries. Extra atoms are inserted into the grain boundaries leads to crystal defects (dislocations) and a transformation from tensile stress to compressive stress. The large compressive {0 0 2} strain and the grain boundary dislocations in gold films can be interpreted according to the above growth mechanism of metallic thin film. Further works are performed to study the relationship between the {0 0 2} strain and the grain boundary dislocations and to evaluate the measurement repeatability and accuracy of {1 1 1} interplanar spacing in the gold film. 4. Conclusions This work employed HREM and GPA to investigate the strain states of three gold materials and thus to develop a new calibration standard of TEM. It was revealed that the lattice strain of {1 1 1} was
smaller than that of {0 0 2} for any kind of gold material at the condition of same measuring method. The 0.65% of {1 1 1} strain in the gold film measured by HREM (0.26% measured by GPA) was smaller than the {1 1 1} strain of two gold nanoparticles. It is deduced that the {1 1 1} interplanar spacing of the gold film was suitable for high magnification calibration of TEM and the gold film was potential to be a new calibration standard of TEM. Moreover, the large compressive strain of {0 0 2} in the gold film was interpreted according to the growth mechanism of metallic thin film.
Acknowledgments This work was supported by the National Key Technology R&D Program with No. 2011BAK15B00.
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