Characterization of metal, semiconductor, and metal-semiconductor core–shell nanostructures

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Characterization of metal, semiconductor, and metal-semiconductor core–shell nanostructures Kumud M. Tripathi*, Mickael Castro*, Jean-Francois Feller* and Sumit K. Sonkar** *SmartPlastics Group, Bretagne Loire University, Lorient, France; **Malaviya National Institute of Technology, Jaipur, Rajasthan

3.1 INTRODUCTION The past 2 decades have been witnessed an enormous research concern on diverse nanostructures as key components in almost every aspect of science and technology [1–4]. Nanoparticles based present technologies offer a broad range of potential applications ranging from energy production/ storage [5,6], environmental remediation [7–10] to biomedical applications [11–14]. In particular, metal, semiconductor and core–shell nanostructures (CSN) are the center of attention, since they offer novel properties d­ istinct from their bulk materials [15,16]. A transition in size domain from microparticles to nanoparticles (NPs) imparts significant and extraordinary changes in physio-chemical, optical, mechanical, electrical, and plasmonic properties of materials [17]. Predominately, downsizing increases the s­ urface to volume ratio in nanoparticles (NPs) and consequently enhances the dominance of more surface exposed atoms, thus introduces the quantum confinement effects within the NPs [11]. Nanostructures having the heterogeneous composition like CSN can provide a new perspective and are attracting an immense interest from both scientific and technological community [18]. The term core–shell was first devised in 1990 during the fabrication of concentric multilayered heterogeneous NPs [19]. Drifting with the current advancement in nanotechnology, the terminology of CSN can be significantly defined for a specific class of heterogeneous nanostructures clumped together with a distinct boundary between cores (inner material) and shells (outer layer material) as long as they can be separately identified [15]. CSN

Metal Semiconductor Core–Shell Nanostructures for Energy and Environmental Applications. http://dx.doi.org/10.1016/B978-0-323-44922-9.00003-X Copyright © 2017 Elsevier Inc. All rights reserved.

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52 CHAPTER 3  Characterization of metal, semiconductor, and metal-semiconductor core–shell nanostructures

comprise of at least one integer active sites and a set of Miller indices (hkl) that defined the high-index crystal facets surrounding on a concave pore surfaces at the center [20]. Depending upon the size to thickness ratio of core–shell, CSN could vary in their shape, size, and surface morphologies. Such as, spherical, hexagonal, concentric, centric, prism, disk, cube, ring, and tubular likes CSNs. Particularly, bimetallic core@shell NPs linked to a broader range of applications in comparison to metallic nanoparticles (constitutes only a single metal) due to the notable tuning of their advantages properties from one material to another [18,21–26]. The active and precise control over the optical, structural, magnetic, and electronic properties of CSN is the key parameter for making them best suitable for specific applications including fabrication of next generation energy and tracking devices. For instance, optical properties of Au–Ag CSN greatly depend upon the core material, either Au or Ag is in the core or shell and similar with the thickness of core and shell material [27,28]. Optical properties of semiconductor and CSN is susceptible to the downsizing the particle size and very much sensitive of the localized surface plasmon [29]. For the CSN, Landes and coworkers recently demonstrates the active control over the optical properties of Ag–AgCl nanostructures via tuning the morphology of narrow interparticle gaps and hence plasmon resonances [29]. More significantly, CSN exhibit the synergistic properties of both the core and shell material and may offer some new distinct properties [11,15,30]. To enable the prospective use of CSN for diverse applications, thoroughly investigation and characterization by various spectroscopic and microscopic techniques are important to explore their wide potential and for optimization of synthesis parameters as well. As most of their unique properties are arise as a consequence of active and differential changes via chemical and­ structural arrangement [31]. Characterization of CSN requires advanced high magnification characterization tools and skills to attain in depth analysis. Both the core and shell are of different composition and one is completely buried inside the other, thus cannot be characterized by single technique and need the combination of classical and advances techniques for complete analysis [11]. A detailed-accurate characterization holds significant potential for unraveling the mechanistic prospects. Based on which physiochemical process for CSN can be tuned for efficient synthesis and further applications. In this chapter, an overview of current technological appraisal for the morphological, microstructural, optical, electrical characterization of metal, semiconductor, and CSN are discussed. Understanding the fate structureproperties relationship of CSN assists their diverse applications. The ­perceptive of CSN nucleation and growth processes with representative

3.2 Microscopic characterization 53

characterization techniques are briefly mentioned with recent examples based on structure-properties relationship. Along with the significant development and progress in the field of characterization techniques used for the investigation of CSN are summarized.

3.2  MICROSCOPIC CHARACTERIZATION Nanoscopic characterizations with high resolution imaging techniques using electromagnetic radiations of shorter wavelength are presently in use for the detailed characterization of nanostructures. Diverse morphological characterization can be easily done with the help of various microscopic techniques. Such as scanning electron microscopy (SEM), field-emission SEM (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), high resolution-TEM (HRTEM), and scanning probe microscopy (STM).

3.2.1 SEM/FESEM Size and shape analysis of nanostructures in a broader study area are carried out with SEM and FESEM. Scanning the sample surface with electron beams in a raster scan pattern in SEM provides the information about surface topography and composition of nanostructures, when equipped with EDAX (energy dispersive X-ray analysis). The basic concept of SEM involves the analysis of backscattered and secondary electrons that are emitted from the samples [32]. Hence can be easily access the location of secondary and tertiary nanostructures. Nanostructures having complicated topography can also be characterized with SEM via a characteristic three-dimensional (3D) appearance due to the relative narrower electron beam and greater depth of field analysis. A detailed SEM analysis provides information about the homogenous or heterogeneous nature of nanostructures, as well as in-depth investigation on their degree of aggregation. FESEM has the advantage of higher magnification in contrast to conventional SEM. FESEM may provides the information about shell surface, whether smooth or rough, but not up to mark as been taken via AFM. FESEM consist of field emission gun (FEG) as an electron source in contrast to tungsten filament cathode in a regular SEM. FESEM exhibited the advantage of low operating voltage and low vacuum conditions for imaging, and is capable of generating high primary electron brightness and smaller spot size for the better resolution of sample images. Fig. 3.1 shows the morphological characterization of Co3O4 nanowires (NWs) and Co3O4/NiO core–shell NWs arrays [33]. SEM images demonstrate the rod like morphology of Co3O4 nanostructures and coating of NiO nanoflakes on Co3O4 NWs. The coatings of NiO nanoflakes

54 CHAPTER 3  Characterization of metal, semiconductor, and metal-semiconductor core–shell nanostructures

■■FIGURE 3.1  SEM images of (A–B) Co3O4NWs arrays and (C–D) Co3O4/NiO core–shell NWs arrays grown on FTO (fluorine-doped tin oxide glass) substrate. (E–F) Enlarged view of the core–shell NWs showing the flake like morphology of the NiO [33].

on Co3O4 NWs are heterogeneous in nature with rough surface. However, the true representation of bulk NPs and discrimination of nanostructures from substrate are the major barriers of the SEM/FESEM. Discrimination of the core from the shell is difficult in SEM analysis, due to the generation of only surface image via the collections of secondary electrons.

3.2.2 AFM AFM are used for the analysis of surface topography with height (Z-axis) profile analysis. AFM is a promising technique for the visualization of

3.2 Microscopic characterization 55

changes in shell thickness or any other relevant changes in physiologically appropriate conditions (i.e., without vacuum and in aqueous medium) that is the prerequisite of electron microscopic techniques. It also permits molecular scale resolution analysis for biological samples. The AFM analysis of sample is based on the scanning and sensing of the topography of a sample surface with the use of near-field microscopy [34]. The distance between sample and image in electron and the optical microscope is comparatively larger than the photon and electron wavelength employed for analysis, so termed as “far-field microscopes” [35]. The AFM analysis (a near-field microscopy technique) is sometimes bit better in comparison with others, concerning the issues that were raised with the inadequate diffraction-related resolutions [36]. AFM offers an excellent resolution, due to the direct vicinity of sample surface to probe and only intensity is recorded with cut off interference signal [36]. Adijanto et al., demonstrated the distribution of Pd@CeO2CSN as a monolayer film grafted onto the alkyl-functionalized oxide surfaces of YSZ (yttria-stabilized zirconia) and pristine YSZ [37]. The comparative AFM images along with line profile diagram for Pd@CeO2CSN and Pd NPs deposited on the surface of clean and alkyl-silanated YSZ(100) are demonstrated in Fig. 3.2. Well dispersed layer of CSN on YSZ (100) surface can be clearly seen with AFM images. The heights of the CSN analyzed with line profile diagram were 4 and 6 nm in range and further showed the formation of monolayer onto YSZ surface. The author also evaluated the thermal stability of CSN on the basis of AFM characterization, Fig. 3.2 (B–D) demonstrated the Pd@CeO2 CSN and their resistance to sintering. Restriction of the possible maximum scan area of samples (typically 100 µm) and interactions between cantilever and sample from uneven surface distribution, analysis of sticky samples are the major restrictions associated with AFM.

3.2.3 TEM/HRTEM Internal nanostructural characterization, discrimination between core and shell material, core size, shell thickness and composition (when equipped with EDAX) of CSN are analyzed with TEM. Deflected and nondeflected transmitted electrons, backscattered and secondary electrons, and emitted photons are analyzed after passing the samples form two dimensional image projections. The primary requirement is just the electrons should transmit from the sample. Energy loss in this process is not significant due to negligible sample thickness [32]. HRTEM is employed for getting the most discrete information about the nanostructure, such as lattice fringes, d-spacing and crystallinity of the nanostructures. Based on this we can easily differentiate between amorphous and crystalline materials, doped

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■■FIGURE 3.2  AFM topography images with representative line scans for Pd@CeO2 and Pd nanoparticles deposited on clean and alkyl-silanated YSZ(100). Panel (A) corresponds to Pd@CeO2 deposited on pristine YSZ(100) calcined in air at 723 K. Panels B–D correspond to Pd@ CeO2 deposited on alkyl-siloxane functionalized YSZ(100) after calcination in air at 723 K (B), 973 K (C), and 1373 K (D). Panels E, F, and G corresponds to Pd nanoparticles deposited on pristine YSZ(100) after calcination in air at 723 K (E), 973 K (F), and 1373 K (G). Comparison of the images for the Pd@CeO2 and Pd nanoparticles clearly demonstrates the high thermal stability of the Pd@CeO2 nanoparticles [37].

verses undoped, and mono/bimetallic nanostructures that cannot be possible via SEM/FESEM and AFM techniques. Fig. 3.3 illustrates the phase transition and growth of silicon NWs on stainless steel surface by TEM, HRTEM, and selected area diffraction (SAED) with different growth time intervals [38]. Fig. 3.3 (A–B) reveals the crystalline

3.2 Microscopic characterization 57

■■FIGURE 3.3  (A) TEM and SAED images of NWs grown for 10 min; (B) HRTEM image of a NW grown for 10 min; (C) TEM and SAED images of NWs grown for 20 min; (D) HRTEM images of a NW grown for 20 min; (E) TEM and SAED images of a NW grown for 40 min; (F) HRTEM images of a NW grown for 40 min [38].

nature of NWs with little amorphous shell. Fig. 3.3 (C–D) clearly reveal the appearance of a thick layer of amorphous shell by TEM/HRTEM characterization. At the increasing growth time, thickness of shell increases as demonstrated in Fig. 3.3 (E–F). Few shortcomings associated [39] with TEM/ HRTEM analysis are the damage/burning of the sample (nanostructures) during high energy beam irradiation and overlapping of images as well.

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Inadequate (smaller) sample analysis area showing only over tens to a few thousand of NPs, sample thickness (preparation of electron transparent sample from the bulk) that should be in nanometer for transmitting the electron from sample that confine its uses for the analysis of especially polydisperse and thick samples.

3.2.4 STEM The structural behavior of nanostructures can efficiently recognized with the invention and use of aberration-corrected lenses, which increases the image resolutions up to 1 Å level [40] and this can be easily achieved by STEM technique. It is an advanced technique and usually connects with electron energy loss spectroscopy (EELS) with the better visualization of clearer image at high resolution. STEM provided the forward motion for better discrimination between the nucleus and shell of nanostructures in contrast to TEM/HRTEM analysis. The implementation of aberration correction in STEM makes possible to enhance single atom sensitivity and improvement in signal to noise ratio in compare to TEM [41]. Also smaller probe and increased probe-forming aperture has significant advances to extract information in 3D even in bulk [42]. Whether, single-crystal structure, twins, or stacking faults can also be recognized on the interface between layers using aberration corrected TEM microscopy. For instances structural composition relation for three layers Au/Pd CSN using aberration-corrected STEM at high-angle annular dark-field (HAADF) imaging mode for direct information on atomic positions were reported by Ferrer et al. [43]. Fig. 3.4A illustrates an ultrahigh-resolution HAADF image of Au/Pd NPs while outer shell image is shown in Fig. 3.4B showing interplanar distance of 3.97 Å. Absence of any extra diffraction spots confirms the lack of twinning or defect free interfaces of core and inner/outer shell and hence reveals the coherent interfaces of Au/Pd nanoparticles. The orientation of different crystals as depicted from the deviation of ideal cuboctahedral shape along with their corresponding intensity profile is demonstrated in Fig. 3.4C–G. EELS was used for the analysis of elemental distributions in lanthanides CSN, when hard to differentiate by diffraction contrast having very close crystalline structures and compositions [44]. The measurement of precise shell thickness is still difficult due to the significant damage of beam or bombardment on irradiation of electron beam [45]. The medium energy ion scattering (MEIS) technique, provides the advancement of ∼108 times higher sample area analysis, in contrast to TEM for better delineation of core size, plate thickness, stoichiometry [46]. In addition, local analyses of CSN for their composition even at subnanometer

3.2 Microscopic characterization 59

■■FIGURE 3.4  Aberration-corrected STEM images. (A) Atomic-resolution HAADF-STEM image of a cuboctahedral Au/Pd nanoparticle. The contrast of the three distinct regions can be clearly seen. Bright dots represent atomic columns. The inset corresponds to the fast Fourier transform of the nanoparticles; (B) Enlarged image of a small part of the exterior layer of NPs (indicated as a white square in A), exhibiting <110> crystal orientation. Image of Au/Pd NPs is showing the deviation from ideal cuboctahedral morphology and its corresponding intensity profile along different crystal orientations. By employing aberration-corrected HRTEM under (C) bright field and (D) HAADF-STEM modes, the atomic positions and chemical species can be clearly analyzed. The scale bar corresponds to 2 nm in both cases. The intensity profiles for (E) the first and (F) the second atomic layers of the bimetallic nanoparticles along the <100> surface and (G) the <111> face show a Gaussian distribution [43].

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resolution are possible with MEIMS [46]. Au@Ag CSN having polydisperse nature both in terms of morphology and composition are successfully investigated with the combination of MEIS and TEM [46]. In the recent past year Lashkova et al., investigated the capabilities of STM microscopy for the estimation of the energy band structure of semiconductor nanostructures by current–voltage(I–V) measurements in local regions using LabVIEW software [47]. They demonstrated that a broad class of nanostructures can be characterized by this technique, which is hard to analyze by conventional methods due to the destruction of material on applying mechanical or thermomechanical stress.

3.3  SPECTROSCOPIC CHARACTERIZATION 3.3.1 XES/XANES X-ray spectroscopy involves the transition from ground state to excited state in XAS (X-ray absorption spectroscopy) or probes the decay process in XES (X-ray emission spectroscopy), assumed as ideal method for the description of chemical nature, oxidation states of metallic NPs and environment of associated atoms in molecules [48]. X-ray absorption near-edge structure (XANES) has been intensively used for the characterization of electronic properties of diverse nanostructures. Redox behavior, differential oxidation states, and the plasmonic activity can easily been characterized with XANES [49]. Unoccupied electronic densities above the Fermi level are probe in XANES along with the local structure. Particularly in the case of CSN, XANES employed to get the information about the charge transfer effect and fingerprints of oxidation state. Principally, whenever the absorption energies are greater than threshold energies of the electron, the fine structure can be studied with X-ray absorption fine structure (EXAFS). K-edge spectra represent the absorption discontinuity originated from a 1 s core level, while L-edge began from 2 s core level at particular X-ray photon energies [48]. Despite experimental difficulties L-edge spectroscopy is presently the more sensitive to surrounding environment than K-edge spectroscopy [49]. In combination, EXAFS and XANES are employed for the complimentary structural characterization, such as used for the analysis of interatomic distances, size symmetry, and local disorders resided over the surfaces of nanoparticles [18]. Structural variations can efficiently tune the electronic and optical properties of nanostructures, and these can thoroughly been investigated in details by XANES and EXAFS spectroscopy. For instance, the mechanism for the enhancement of quantum yield of CdSexS1−x quantum dots (QDs) on increasing sulfur content during one injection route synthesis was explored with XANES and EXAFS analysis [50]. Cd K-edge and the Se K-edge XANES spectra for CdSexS1−xQDs are demonstrated in Fig. 3.5A–B

3.3 Spectroscopic characterization 61

■■FIGURE 3.5  XANES at (A) Cd K-edge and (B) Se K-edge for variable x of CdSexS1−x. Schematic illustration of (C) Zinc blended structure of CdSe and CdS crystal. In the zinc-blended structure, Cd atoms are located on the tetrahedral sites of a face-centered cubic structure, which consists of Se atoms. From XAS analysis results, three different cases were speculated for synthesized CdSexS1−x QDs: (D) CdS/CdSe core–shell model; (E) CdSexS1−x homogeneous model and (F) CdSe/CdS core– shell model [50].

respectively with reference compounds Cd foil and Se foil. The white line energy of the XANES spectrum of CdSexS1–x QDs in Cd K-edge was in between CdS and CdSe sample while in Se K-edge it was almost the same. It reveals that both the regions are clearly separated with two strong bonding regions, one between Cd and S and other between Cd and Se. The possible structural models for CdSexS1–x QDs are demonstrated in Fig. 3.5D−F having zinc-blende structure and the coordination number 4 for both NCd–Se(S) and the NSe–Cd (Fig. 3.5C) [50]. However, inability to discriminate between scattering atoms having a slight difference in their atomic number and lowresolution limit are still the major limitations of current X-ray techniques.

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3.3.2  Raman and surface enhanced Raman spectroscopy Information about the vibrational and electronic structures of semiconductor NPs and CSN can easily be obtained using Raman spectroscopy. Raman spectroscopy is recognized as the most influential method among the recent characterization techniques and is based on the inelastic scattering of visible light by the nanostructural materials [51–53]. The energy of irradiated and scattered light for both Stokes and anti-Stokes radiations are closely related with molecular vibrations of nanostructures. Molecular level information related to the nature of chemical bond and symmetry of molecules could easily be characterized by analyzing the Raman shift (shift in scattered light) [54]. Crystalline order and inplane crystal size of CSN can be conveniently measured by using Raman spectroscopy. Small quantum efficiencies in Raman spectroscopy can be avoided by the use of resonance Raman spectroscopy via the matching of electronic transition energy of nanostructures with excited or scattered photon energy from light beam. Enhancement in vibrational bands can reach up to 108 folds in resonance Raman spectroscopy, and hence, chromophores types of moieties can also be investigated comprehensively in solid state just by tuning the excitation wavelengths of resonance [32]. Surface-enhanced Raman spectroscopy (SERS), basically required and analyzed the excitation of localized surface plasmons is highly employed for the characterization of surface adsorbed NPs to provide a molecular fingerprint spectrum that makes their advanced studies much easier and can easily differentiate between two closely associated atoms [55]. Besides this, SERS is also employed for the characterization of surface processes in CSN containing SERS active metals, such as Au, Ag, and Cu. NPs size has a significant impact on SERS intensity with a combination of excitation wavelength as well (Fig. 3.6A), due to the coupling between NPs LSPR (localized surface plasmon resonance) and excitation wavelength. Bimetallic CSN enhances the SERS in contrast to homogeneous NPs because of the formation of increased electric field between two close metal layers [56]. Enhancement in SERS is proved to be highly advantageous for many potential applications, such as ultrasensitive sensing of possible analytes in complex matrices via a nondestructive approach and biomedical applications. Fig. 3.6B shows the effect of shell thickness (Ag) on adding different concentration of AgNO3 during synthesis process for Au@Ag CSN in SERS spectra [57]. At first, intensity of SERS signal increases with shell thickness and then decreases on addition of 70 µL solution (too thick shell) because of the trapping of scattering light in metal layers. Electromagnetic enhancement explained the enrichment in SERS signals restricted in the hot junctions of Au core and

3.3 Spectroscopic characterization 63

■■FIGURE 3.6  (A) Comparison of the SERS intensity of ring stretching peak (1368 cm−1) for the different Au@Ag CSN at three excitation wavelengths: 532,

633, and 785 nm. All spectra were recorded in solution [56]. (B) SERS spectra of Au@Ag CSN with different thickness of Ag shells, number 1–5: Au@Ag CSN with (1 mM) AgNO3 solution of 0, 30, 50, 70, and 100 mL, respectively [57].

Ag shell [57]. However, full understanding of the SERS spectrum is still a future challenge owing to the complexity and irreproducibility from differently designed plasmonic nanostructures [58].

3.3.3  EDS analysis Energy dispersive spectrometry (EDS) is equipped with microscopic techniques, such as SEM/TEM. EDS efficiently adds the much more accurate information for the detailed analysis of nanostructures, such as detailed mapping of the distribution of elements within a specified region. EDS analysis is employed for the analysis of relative positions of atoms in individual CSN [28]. Thus, EDS serve as a potential tool for the characterization of chemical nature of the core and shell of particular CSN. Compositions of nanostructures regarding carbon, nitrogen, and sulfur contents are carried out with the elemental analyzer.

3.3.4  XPS analysis X-ray photoelectron spectroscopy (XPS) is a significant technique to explore the exact and precise information regarding the surface nature and chemical composition of CSN. XPS analysis is employed for in-depth analysis or to get information about atomic composition. Chemical status of the surface atom, elemental composition of CSN or NPs, empirical formula of nanostructures, electronic state or modes of chemical binding of surface ligands can also

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■■FIGURE 3.7  XPS spectra of Fe3O4@ nSiO2@mSiO2@NaYF4: Yb3+, Er3+ [59].

be analyzed. XPS associated with milling can extract the more and accurate elemental information while moving within an indepth analysis of CSN concerning its core–shell composition. Fig. 3.7 shows the binding energy of O (1s, 532.1 eV), Y (2p3/2, 161.2 eV) (2p1/2, 159.0 eV), F (1s, 685.0 eV), Na (1s, 1073.5 eV), and Si (2p, 103.7 eV), for to Fe3O4@nSiO2@mSiO2@ NaYF4: Yb3+, Er3+ [59]. However, the precise measurements of shells thickness and nature of growth of shell, whether isotropic or not on individual core nanocrystals are still inadequate and provided an ensemble measurement.

3.3.5  FTIR analysis Fourier transformed infrared (FTIR) spectroscopy is used for the characterization of surface functionalization and extent of derivatization [60–63]. Fig. 3.8A represents the presence of carboxylate group on the surface of superparamagnetic Fe3O4 supraparticles (SPs), by showing the characteristic C = O peak at 1711 cm−1, peak at 1562 and 1408 cm−1 for antisymmetrical and symmetrical COO− vibration [64]. FTIR spectrum of Fe3O4 supraparticles@MIL-100(Fe) (MIL = core–shell nanostructures Materials of Institute Lavoisier) in Fig. 3.8C shows the absence of characteristic peak for H3btc (benzene 1,3,5 tri carboxylic acid) (Fig. 3.8B) and new peaks at 1622 cm−1 and 1380 cm−1appears, that corroborate the signature peak related COO− vibrations [64].

3.4 Optical characterization 65

■■FIGURE 3.8  FTIR spectrum of (A) Fe3O4 SPs;(B) H3btc and (C) Fe3O4 SPs@MIL-100(Fe) [64].

3.3.6  Dynamic light scattering DLS is a very efficient analysis applied for the direct measurement of mean particle sizes importantly in solutions or suspensions. DLS provided the hydrodynamic diameter of CSN and generally termed as quasielastic light scattering spectroscopy or photon correlation spectroscopy [11]. Shell thickness can be measured indirectly with DLS by measuring the hydrodynamic diameter of nanostructures before and after coating [65]. Extent of modification of core can be determined indirectly by zeta potential measurement in solution or dispersion [66]. Alterations in zeta potential value before and after modification can confirm the surface functionalization or the formation of NPs based composite. For example, the zeta potential value decreases up to −38.3 mV for DNA-Au-NPs conjugates in contrast to Au-NPs (−29.4 mV) showed a successful modification of Au-NPs with DNA [57].

3.4  OPTICAL CHARACTERIZATION 3.4.1  UV-visible spectroscopy Optical characterizations are used to obtain information regarding the nature of the covering of shell material on the core surface. Resonance between alteration in electric field generated by electromagnetic radiation and collective oscillations of outermost electrons governs the optical behavior of CSN [67]. In consequence, localized surface plasmon resonance (LSPR) significantly affects the optical properties of CSN [67]. LSPR analysis depends upon the morphology, dielectric constant of the core and shell materials and chemical

66 CHAPTER 3  Characterization of metal, semiconductor, and metal-semiconductor core–shell nanostructures

nature of surrounding environments [68]. Change in morphology and materials in CSN alter the resonance conditions and consequently resulting in the modification of CSN dispersion or solution. For instance, an aqueous dispersion of Ag nanoprism showed blue color while, grey color was observed for Au nanorings and brown for Au–Ag core–shell nanorings [68]. Absorption spectroscopy characterizes the nanostructures with absorption capacity in UV-vis region via individual absorbance of the core, shell, and core–shell materials. A single absorption band in the UV-vis region is a signature mark showing for the fabrication of bimetallic alloyed nanostructures rather than CSN [69]. Since two homometallic nanostructures possess different plasmon resonance energy directly related to their composition [69]. The occurrence of two distinct plasmons (collective excitations of conduction electrons) resonance peaks is characteristic for the formation of CSN [70]. A change in intensity was observed with varied shell thickness along with visual shift at peak position in absorption spectrum. A change in absorption band for the two different metallic NPs and CSN are also recognized. Optical absorption spectra can also be used to probe the size of nanostructures and estimated band gap energy using the simple power law [71]. For instance, in the particular case of Au@Ag NPs consist of two distinct plasmon absorption bands, shell thickness was precisely determined [72]. The intensities of corresponding absorption bands significantly depend on upon the variation of shell material (Ag), and with increase in shell thickness absorptions bands are shifted to longer wavelengths. However, the optical contribution of core material in CSN having thick shell may be adequately screened [73]. In-depth analysis of the distributions of different plasma modes in CSN and their relationship with structural variation is essential for the modulation of near-field conduct and further engineer the plasmonic analogs as radio frequency antennas. The presence of Au core in Ag–Au–Ag nanowires has a marginal role in determining the nature of plasmon and their spatial distribution [74]. Mayer and coworkers characterized the “controlled living nanowire growth” using linear growth rate of Ag on Au cores and fabricated monodisperse Ag–Au–Ag NWs [74]. UV-vis-NIR extinction spectra was recorded at different reaction times for varying NWs length as shown in Fig. 3.9 (a top) and corresponding electromagnetic simulations with boundary-element method (BEM) are illustrated in Fig. 3.9 (a bottom). Further comparison of the wavelength width of the analyzed and calculated spectra was showed zero order kinetics for the growth of NWs on the controlled addition of silver. Core–shell NWs was characterized by getting a linear slope on plotting resonance wavelengths on one axis including all longitudinal plasmons and degree of silver elongation (AgEn) as denoted by molar ratio

■■FIGURE 3.9  (A) Upper panel: Vis-NIR spectra recorded during silver growth; the PT (penta twinned) Au NR core is displayed as a dotted curve; lower panel: calculated (BEM) extinction spectra of Ag–Au–Ag (solid curves) and pure Ag (dashed curves) NWs with dimensions corresponding to the experimental ones; (B) resonance wavelengths for the dipolar and second through fifth order multipolar modes versus AgEn. The aspect ratio is also plotted for reference. Solid lines are linear fits to the data. The position of the corresponding modes for the PTAu nanorods cores are plotted as open symbols; (C) avelength shift of the dipolar plasmon mode for growth with a faster addition rate, and for the standard rate on PT Au NRs with different dimensions (180 × 34 nm, 180 × 32 nm, 210 × 32 nm). The dashed line represents a theoretical estimate, using the silver-to-gold volume ratio in one nanowire as AgEn. The open circle is the common origin; (D) Spatial distribution of plasmon modes for a short Ag–Au–Ag NW (AgEn = 0.5) as measured by EELS; (E) Spatial distribution of plasmon modes for a long Ag–Au–Ag NW (AgEn = 5.3) as measured by EELS [74].

68 CHAPTER 3  Characterization of metal, semiconductor, and metal-semiconductor core–shell nanostructures

of added silver to gold seeds on another axis as shown in Fig. 3.9B. Faster addition of Ag resulted in a nonlinear shift (Fig. 3.9C) due to the faster increase of AgEn than the plasmon band shift. The nature of plasmonic modes and their number in the whole UV−vis-NIR region significantly depend upon the length of NWs (Fig. 3.9D–E) rather than nature of Au core as investigated by EELS.

3.4.2  Fluorescence spectroscopy Optical analysis concerning the characteristics of CSN is easily analyzed by comprehensive photoluminescence (PL) measurements. The nature of the excitations and thus obtained emissions govern the optical properties of nanostructures [10,75–79]. Semiconductor CSN offers strong PL emission in the visible region (termed as surface plasmon resonance), since it offers coherent oscillation of electrons in conduction band, when interact with electromagnetic waves [80]. The PL emissions are closely associated with physical and chemical properties of their surfaces [81]. Therefore, the detailed evaluation of physical dimensions of a core and shell in the strong confinement regime of optical properties of CSN hinges upon explicit verification for enhancing its PL efficiencies. Their high fluorescence quantum yield with narrow and symmetric spectrum emerged CSN as an alternative to conventional dyes in biomedical imaging [82]. CSN within 2–8 nm in diameter are termed as QDs. QDs exhibited somehow atom like optical behavior, with very much facile synthetic process. The quantum efficiency of CSN QDs are higher regarding increased quantum yield than metallic NPs due to the lessening of response time by introducing core material with selective shell materials coating. More significantly optical properties of QDs can be tuned with size due to quantum confinement over the entire visible spectrum [81]. Coatings of shell material in CSN offers generation of new surface energy traps and in consequence lead to generations of newer emission bands and further recombination of electrons and holes in a nonradiative way causing the emissions [83]. Alterations in size, shape, morphology, or distribution of NPs have considerable impact on the PL properties of CSN. As, in case of CdSe-ZnS QDs, a nonlinear optical absorption was recorded when ZnS shell thickness was increased due to the larger number of electron-hole pairs and hence higher free carrier concentration [84]. The occurrence of a new emission band, in contrast to CdSe QDs was observed in CdSe-ZnS, which contributed to the generation of the new surface trap. With the increase in shell thickness (ZnS) optical limiting threshold was also noticed to be decreased [84]. In another report, the emission of ZnO–ZnS CSN was reported to green emissions along with a blue emission in contrast to ZnO NPs [85]. Size distributions can also be measured by analyzing full width

3.5 Other spectroscopic characterization 69

at a half maximum (FWHM) value through computational Gaussian curvefitting algorithm of the PL emission peaks. Smaller FWMH stands for the narrow size distributions of the nanoparticles and larger FWMH giving the information of a wide range of size distributions. QDs having the ability of upconversion for converting infrared light to visible light with high photostability have been a research hotspot because of their ability to offer deep tissue imaging [30,44,59]. Lanthanide-doped upconverting nanoparticles (UCNPs) convert excitation light of NIR (near infrared) to visible emissions via the consequence of a stepwise photon mechanism and its intensities are significantly related to the shell thickness of CSN [86]. Zhang et al., investigated the dependence of shell thickness of NaYF4:Yb, Er@NaGdF4 CSN on enhancement of upconverted emissions in a linear relationship [45]. They also reported that the intensity of upconversion emission and lifetime had more resistant toward quenching with moisture or water molecules [45]. Fig. 3.10 demonstrated the optical characterization of (α-NaYbF4:Tm3+)/CaF2 NPs having upconversion properties [87]. The intensity of upconverting emissions exhibited from the (α-NaYbF4:Tm3+)/CaF2 CSN was ∼35 times higher in contrast to core α(NaYbF4:0.5% Tm3+) alone NPs. Fig. 3.10B clearly showed the difference in intensity by analyzing the cuvettes images having both core NPs and CSN suspensions respectively at 975 nm excitation. The visible emissions from CSN at 975 nm excitation can also be clearly demonstrated by Fig. 3.10B. CSN also exhibited longer lifetime (300 µs) in contrast to core NPs (90 µs) as demonstrated in Fig. 3.10C. Characterization using optical microscopy is proven not be adequate to obtain a comprehensive optical response at the single particle levels. Low resolution (0.5–0.2 µm) of the optical microscope is also another drawback for resolving the spatial features. Intrinsic optical resolution and size of the fluorescent labels limited the effective image resolution usually down to only a submicron or a few hundreds of nanometers. Particle-by-particle basis characterization of nanostructures can be employed by using linear optical microscopy at the near-field or far-field. A time-resolved optical responses of particular CSN can be quantified by using nonlinear optical microscopy (four-wave mixing (FWM) microscopy) on ultrafast time scales [88].

3.5  OTHER SPECTROSCOPIC CHARACTERIZATION Thermal stability of nanostructures and sometimes the extent of functionalization are examined by thermal gravimetric analysis (TGA) as a function of temperature. A quantitative composition of CSN is determined via the weight loss versus temperature in the inert atmosphere. Since decomposition

70 CHAPTER 3  Characterization of metal, semiconductor, and metal-semiconductor core–shell nanostructures

■■FIGURE 3.10  Optical characterizations of (NaYbF4:Tm3+)/CaF2 core–shell UCNPs (hexane suspensions). (A) upconversion PL spectrum

under laser excitation at 975 nm; (B) photographic images of cuvettes with suspensions of the core and the CSN under laser excitation at 975 nm; (C) decays of PL at 802 nm for the α-NaYbF4:0.5% Tm3+ core and the α-(NaYbF4:0.5% Tm3+)/CaF2 CSN; (D) up-conversion PL spectra of 27 nm α-(NaYbF4:0.5% Tm3+)/CaF2 CSN and 100 nm β-NaYbF4:0.5% Tm3+ (hexagonal) NPs when excited by a 975 nm CW diode laser at a power density of ∼0.3 W/cm2. The insets in (A) and (D) show the absorption spectra of UCNPs (normalized at the PL excitation wavelength for the 2F7/2 to 2F5/2 transition of Yb3+ ions) [87].

temperatures of organic cores are lower than an inorganic shell, hence, TGA analysis frequently used to support and analyze the fabrication of a hollow organic core [11,89]. Crystalline and amorphous nature of semiconductor core–shell or metallic nanostructures are characterized via powder X-ray diffraction (XRD) analysis [9]. Indeed XRD offers the information about preferred orientation of nanostructures in polycrystalline or powder sample in the solid

3.6 Conclusions 71

state. In addition crystallographic structure and crystalline size (grain size) can also be analyzed with XRD via broadening or intensity change of the peak. XRD has long been known to characterize the presence of core–shell morphology in the diverse NPs by analyzing the presence of separated phases for both the core and shell simultaneously [15]. Diffraction peak intensity can also determine the nature of shell coating on core material (homogeneous/heterogeneous) in CSN. After the coating diffraction peak intensity of base material decreases with increase in core thickness and after sufficient depth it disappears [11]. If shell material is amorphous in nature, then low-intensity diffraction peak for the core elements appears. XRD analysis can also make identification of unknown nanostructures. For example, the fabrication of brick-like nanoparticles (BLNs) of Ag@ Fe3O4 for potential applications in nanomedicine was differentiated from conventional Ag–Fe3O4 CSN with comparative XRD analysis [90]. Ag@ Fe3O4 CSN demonstrated the sharper peak than BLNs because of fewer crystallographic ordering having a smaller Ag particle. Fig. 3.11 demonstrated the characterization of Au–Cu2O, Pt–Cu2O, Pd–Cu2O, and Ag–Cu2O CSN via XRD analysis [91]. Both the CSN exhibited three main diffraction peaks that corresponds to Cu2O, (110), (111), and (200), and two diffraction peaks, (111) and (200), which corresponds to metal core, which confirms the growth of Cu2O shell on metal core. Weaker intensity of diffraction peak for metal core corroborated as a result of preferential deposition of shell material on core material.

3.6 CONCLUSIONS In summary, this chapter features an overview of the presently used characterization techniques focusing on a new dimension of CSN. A meticulous investigation of morphological and chemical constituents of CSN is very much indispensable for the precise purpose of their physiochemical properties. The breakthrough discoveries in the synthesis of CSN required its proper characterization. A precise characterization of CSN is critical to achieving a better view of the intricate relationship between their structure and functionality for nanoscale mechanisms to apply these in next generation devices. Not simply a single or pair of techniques can have the whole characterization scenario. In-depth analysis requires a combined and efficient function of many spectroscopic and microscopic techniques to understand better about the system. Several techniques are presently working fine, but also cause some limitations and need to couple with other characterization techniques. Even the long ways have to go

72 CHAPTER 3  Characterization of metal, semiconductor, and metal-semiconductor core–shell nanostructures

■■FIGURE 3.11  XRD patterns of (A) Au-Cu2O, (B) Pt-Cu2O, (C) Pd-Cu2O, and (D) Ag-Cu2O [90].

forward to overcome the inadequacies of nanoscale characterization for CSN and its correlation for plasmonic applications. The accurate characterizations of nanostructures consequently lead to a decisive understanding of linked properties for diverse, complex CSN in future technological applications.

ACKNOWLEDGMENT K.M.T. thanks European MEET (Material for Energy Efficiency in Transport) program, for a postdoctoral research fellowship and UBS (University of South Brittany) for infrastructure. M. C. and J.-F. F acknowledges the European MEET program for funding. S. K. S thanks DST New Delhi for funding [SB/EMEQ-383/2014].

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