Review
Optical properties and applications of hybrid semiconductor nanomaterials

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Abstract

Hybrid semiconductor nanomaterials (HSNs) possess unique and interesting optical properties and functionalities that find important applications in emerging technologies. Compared to single component nanomaterials, hybrid nanomaterials offer the possibility and flexibility to control their properties by varying the composition of the materials and related parameters such as morphology and interface. Hybrid nanomaterials are essentially composite materials with relevant physical dimensions for the interface region between different components on the atomic up to nanometer scales, the same length scale of nanomaterials. This article provides an overview of some of the fundamental optical properties of hybrid semiconductor nanomaterials as well as their exploitation for potential applications in different fields. A number of examples from recent research are discussed to illustrate the points of interest and to highlight the salient features of HSNs.

Introduction

Nanomaterials have interesting properties and useful functionalities that can differ substantially from their bulk counterparts. For instance, spatial quantum confinement effect results in significant change in optical properties of semiconductor nanomaterials. Likewise, the very large surface-to-volume (S/V) ratio has major influence on their optical and surface properties. As a result, semiconductor nanomaterials have attracted significant attention in research and applications in areas including energy conversion, sensing, electronics, photonics, and biomedicine. Parameters such as size, shape, and surface characteristics can be varied to control their properties for different applications of interest.

For many applications, it is highly desired to be able to control and alter the properties and functionalities of nanomaterials with greater flexibility and possibility. One approach is to use hybrid nanomaterials that have properties different from those of single component nanomaterials. The use of multiple components offers a higher degree of flexibility for altering and controlling properties and functionalities of nanomaterials. Hybrid nanomaterials can be generally defined as nanomaterials that contain more than one single component. Examples include doped nanomaterials and composite nanomaterials. For many emerging technologies, hybrid nanomaterials with improved optical and electronic properties are needed, including solar energy conversion, optical devices, optical imaging, and biomedical detection and therapy.

Hybrid nanomaterials have many different useful properties and applications. In this review, we focus on optical properties and related functionalities and applications. In principle, the constituting components can be insulators, semiconductors, or metals. The components can also be classified as inorganic, organic, or biological. Due to space limitation, we will further restrict ourselves to hybrid nanomaterials that contain an inorganic semiconductor as one of its components. Thus, the emphasis is on optical properties and applications of hybrid semiconductor nanomaterials (HSNs).

The different components in HSNs have apparently different chemical and physical properties. Following the conventional classification of materials as metal, semiconductor and insulator, one can have hybrid nanomaterials with two components of metal–semiconductor, metal–insulator, semiconductor–insulator, and so on. They could also be composed of two different metals or two different semiconductors, etc. Of course, a hybrid system can also be composed of multiple components.

The difference in optical properties between hybrid nanomaterials and their constituent components depends largely on the chemical nature of each component as well as how the two or more components interact with each other. The interaction between the components depends strongly on characteristics such as interface, size, shape, and structure. In the extreme case of no or little interaction between the components, the optical properties of the composite should be equivalent to a simple sum of the optical properties of the individual components. In cases where the interaction between the components is strong, the optical properties of the composite system can differ substantially from the simple sum of the properties of the individual components. The characteristics of the individual components are lost and new feature arise as a result of the strong interaction. For example, new absorption and emission bands may appear in the absorption and emission spectra of hybrid nanomaterials as compared to the individual components. This can be rationalized by considering the interaction as a perturbation and the starting individual components as the zeroth-order system.

For example, in a simple two-component system consisting of two different materials A and B, their interaction can be considered as a perturbation or coupling energy V. If the zeroth-order wavefunctions are labeled as ψa and ψb and the new composite system wavefunctions labeled as ψ1 and ψ2 for the lower and higher energy states resulting from the two initial zeroth order states, applying perturbation theory then gives us the following:ψ1=c1ψa+c2ψbψ2=c1ψa+c2ψbThe coefficients in the above two equations depend on the strength of perturbation V. This is similar to the treatment of molecular orbitals for a heterodiatomic molecule from atomic orbitals of two different atoms. In general, the stronger V is, the more different the new states from the zeroth-order states. Fig. 1 shows a schematic of the energy level involved in such a simple two-component system.

Most hybrid systems are featured with moderate interaction. In such cases, the optical properties of hybrid materials may still resemble those of the individual components while some changes may occur or new features may arise, but usually not to the degree as in the strongly interacting case. For example, absorption and/or emission bands of the hybrid system will likely exhibit modest changes compared to those of the original components, e.g. spectroscopic shift or line width broadening.

Since this review focuses on inorganic hybrid semiconductor nanomaterials, the systems to be discussed will have at least one component that is an inorganic semiconductor nanomaterial. We will exclude alloys that are atomic level hybrid materials. The hybrid systems to be discussed are made up of at least two components and each composite retains its chemical identity with physical dimension on the few to a few hundred nm scales. We further restrict to situations where at least one component is nanocrystalline, which is considered as the primary component. If non-crystalline components are present, e.g. passivating molecules, they will be considered as secondary components. By doing so, systems that are entirely amorphous, i.e. non-crystalline, on the atomic scale will be excluded. The electronic energy levels or structure in the hybrid system is dependent on the crystal structure of the individual components

Besides energetic properties, inter-component structural or morphological properties of hybrid nanomaterials are also important in determining their properties and functionalities. Fig. 2 shows an illustration of a simple two-component system in which each component is crystalline. Of course, most real hybrid nanomaterials have more complex compositions and structural features, with specific examples given later. The case on the left of Fig. 2 has no long-range order among the constituting nanoparticles while the case on the right does. The two systems illustrated can have quite different properties due to their difference in structure [1].

A special note should be made about the distinction between semiconductors and insulators, particularly when applied to metal oxides (MOs). Some MOs are insulators while others are considered as semiconductors, depending on their bandgaps. There is no clear-cut about what the bandgap energy should be in distinguishing semiconductors and insulators, even though 3.5–4.0 eV is usually chosen somewhat arbitrarily as the dividing point [1]. Materials with bandgap larger than this number are often considered as insulators while those smaller semiconductors. Sometimes, a given MO with a fixed bandgap can be considered as insulator or semiconductor, depending on the relevant context. Also, the bandgap energy could differ depending on if it is defined optically or electrically. For example, TiO2 and ZnO are considered as insulators optically in most contexts due to weak or no visible absorption. However, they are often considered as wide bandgap semiconductors also because of their electrically semiconducting properties usually caused by doping. Furthermore, when they are hybridized with a small bandgap semiconductor such as ZnSe or metal such as gold, it is more convenient to treat TiO2 and ZnO as insulators since the optical and electrical properties for such systems will be dominated by the small bandgap semiconductor or metal. In this article, we will consider MOs mainly as insulators since the focus is on optical properties and application. In some context, it is more appropriate to consider some MOs, such as TiO2 and ZnO, as semiconductors and proper justification will be given when this occurs.

In the interest to save space and stay focused, this review will not provide a detailed discussion of synthesis and characterization of nanomaterials including HSNs. However, a brief discussion is given to make the text self-contained and better connected to the discussion of optical properties and applications later. Synthesis of HSNS is presented next with characterization presented in the following section.

Methods for synthesis and fabrication of nanomaterials can be roughly divided into and gas- or vapor-based (chemical or physical) and solution-based (usually chemical). Gas- or vapor-based techniques include chemical vapor deposition (CVD), metal–organic CVD (MOCVD) [2], [3], [4], [5], [6], [7], [8], [9], [10], and molecular beam epitaxy [11], [12]. Solution-based techniques include precipitated hydrolysis, melting and rapid quenching, hydrothermal, and thermal decomposition. In solution-based synthesis, two critical steps involved are initial nucleation and subsequent growth [13], which apply for metal, semiconductor or insulator. The properties of the nanomaterials produced, e.g. size, shape, and crystal structure, can be varied by controlling the nucleation and growth processes.

For metal nanomaterials, the synthesis usually involves reduction of metal salts or metal ions by appropriate reducing agents in solution. Simple reduction reactions usually produce small nearly spherical nanoparticles [14], [15], [16], [17], [18], [19], [20]. For large nanoparticles, the technique of seed-mediated growth has been developed [21], [22], [23], [24], [25]. For more complex shaped structures, such as nanorods, aggregates, hollow nanospheres, nanowires, nanocages, nanoprisms, and nanoplates, the synthesis is typically more involved, including using surfactants such as cetyltrimethylammonium bromide (CTAB) [26], [27], [28], [29], [30], [31] or galvanic replacement [32], [33], [34], [35], [36], [37].

For semiconductor nanomaterials (quantum dots, QDs or nanocrystals, NCs), the synthesis methods vary depending on the nature of the semiconductor. For semiconductors such as Si and Ge that involve only one element, their synthesis usually involve reduction of a salt of the corresponding element with appropriate reducing agent or decomposition of precursor compounds, which is similar to the synthesis of metal nanomaterials [38], [39], [40], [41], [42]. In a typical synthesis of binary semiconductors such as II–VI and III–V semiconductors, reactants or molecular precursors containing the desired cation and anion components are mixed in an appropriate solvent to produce the nanostructures of interest [43], [44], [45], [46], [47], [48], [49], [50]. Surfactant or capping molecules are often used to stabilize the nanoparticles or to direct particle growth along a specific crystal plane to produce nanospherical structures [51]. Nanoparticles in aqueous solutions tend to have a high density of defects or trap states and low overall photoluminescence (PL) yield. Their optical properties, especially PL, are usually sensitive to pH of the solution [52]. Recently, synthesis of semiconductor nanomaterials in organic solvents has produced various high quality nanoparticles in terms of size monodispersity, surface properties, and PL yield. The organic solvents are usually mixtures of long chain molecules, e.g. alkylphosphines, R3P, alkylphosphine oxides, R3PO (R = butyl or octyl), and alkylamines [43]. For instance, highly monodisperse and luminescent II–VI semiconductors such as ME (M for metal such as Cd and Zn and E for S, Se, and Te) have been synthesized by injecting liquid metal–organic precursors containing M and E into hot (around 150–350 °C) coordinating organic solvents [13].

For insulator nanomaterials, the general synthetic strategy is very similar to that used for synthesizing semiconductors. Most insulator nanomaterials are based on binary compounds, e.g. ZrO2, and their synthesis in solution involves reaction between reactants containing the appropriate cation and anion components [53]. Solution methods, e.g. hydrolysis, are most commonly used for making MO nanoparticles or other shaped nanomaterials, e.g. TiO2, ZnO, WO3, and SnO2 [54], [55], [56], [57], [58], [59] As a complementary method, hydrothermal synthesis is also often used for making MO nanomaterials [60], [61], [62], [63], [64], [65], which is particularly useful for making non-spherical particles such as one-dimensional (1D) nanostructures [66], [67], [68].

Specific for HSNs, the synthesis is usually based on either a sequential, step-wise approach or simultaneous, one-pot or one-step approach. In a sequential approach, the different components are synthesized first separately or in different steps, and the hybrid system is produced at the end. This is the more commonly used method. In the one-pot approach, all the reactants are added simultaneously and the different components are produced at the same time.

The sequential approach has often been used to make “core/shell” structures, a class of unique hybrid nanostructures that contain two different types of materials, one as the core and another as the shell. Various combinations of core/shell structures can result from using metal, semiconductor, or insulator for the core or shell, or just two different metals, semiconductors, or insulators for the core and shell [69], [70], [71]. The shell is usually used to alter the properties of the core for different applications of interest. For example, CdSe/ZnS was studied as a system to enhance PL stability and yield of CdSe as well as to lower the toxicity since ZnS is less toxic than CdSe [72], [73], [74]. The shell semiconductor is usually chosen to have larger bandgap and lower toxicity compared to the core semiconductor. For example, CdSe NCs were overcoated with a shell of ZnS [72], [73], [74], [75], [76], [77], [78], ZnSe [79], or CdS [47], [80], which resulted in dramatic improvements in luminescence efficiency of CdSe [73]. More complex structures, such as core/shell/shell, can also be produced, e.g. CdS/HgS/CdS [81], [82]. The detailed crystal structures of the shells are usually not as well identified as the core due to their thinness. In many cases, the shells are polycrystalline. Other examples of core/shell hybrid nanomaterials include CdS/TiO2 [83], CdS/Au [84], TiO2/Pt [85], TiO2/Au [86], TiO2/CdSe [87], CdSe/SiO2 [88], CdTe/SiO2 [89], and Au/SiO2 [90], [91], [92]. In synthesis, the shell is produced after the core in a sequential manner.

Another use of the sequential approach is for surface modification of nanostructures post synthesis. One example is silanization, i.e. coating of a silica layer on the nanostructure surface [93], [94], [95], [96]. Surface modification is often necessary for specific applications. For instance, while QDs synthesized in organic solvents tend to have higher quality in terms of lower density of surface trap states and thus higher PL yield, their hydrophobic surfaces are not compatible with applications that require QDs with hydrophilic surfaces, e.g. biological applications involving aqueous environment. In order to render QDs with hydrophobic surface hydrophilic, a number of strategies have been developed, including ligand exchange [97], [98], [99], [100], silanization [101], [102], [103], and surface coating using amphiphilic polymers or surfactants such as phospholipids [104], [105], [106], [107]. All these strategies for surface modification result in practically hybrid nanostructures with the primary component being a semiconductor, at least in terms of optical properties of interest.

For one-pot approach to synthesizing HSNs, a good example is synthesis of doped semiconductor nanomaterials [108]. Doping refers the process of intentionally introducing a small amount of a foreign or different element into a host material. For example, B or N can be introduced into Si as a way to alter its chemical composition and, more importantly, to influence its properties, e.g. optical and electronic. Another major use of doping is in phosphors based on binary semiconductors such as ZnS with dopants such as Cu, Mn, and Ag ions [108], [109], [110], [111], [112], [113], [114], [115], [116], [117], [118], [119]. For doped nanomaterials, the method is often only slightly altered from that used for undoped semiconductor synthesis. The main modification is that the dopant element is introduced during synthesis. For doping to be successful, the dopant needs to be compatible to the host material. Similarly, many MO semiconductors and insulators have been successfully doped with different elements such as Gd3+ [55], N [120], [121], [122], [123], [124], C [125], and Tb3+ [126]. For example, Cu-doped ZnO NCs were synthesized using copper acetate and zinc acetate as the Cu and Zn sources, respectively, through hydrolysis using sodium hydroxide and with isopropanol as a solvent [127].

More complex nanostructures can also be produced but usually involve multiple steps and processes such as self-assembly [128], [129], [130], [131], [132]. Discussion of synthesis of such structures is beyond the scope of this review.

Many techniques have been employed to characterize nanomaterials including HSNs. Since HSNs are more complex than single component nanomaterials, their characterization usually demands a combination of techniques or more sophisticated techniques. In general, the characterizations are based on spectroscopy, microscopy or X-ray techniques.

Optical spectroscopic techniques are widely used in the study of optical properties of different materials including nanomaterials [133]. The different techniques are usually based on measuring absorption, scattering or emission of light that contains information about properties of the materials. Commonly used techniques include electronic absorption (UV–vis), photoluminescence (PL), infrared red (IR) absorption, Raman scattering, dynamic light scattering, as well as time-resolved techniques, such as transient absorption and time-resolved luminescence [133]. Other more specialized techniques include single molecular spectroscopy and non-linear optical techniques such as luminescence up-conversion. These different techniques can provide different information about the molecular properties of interest, including energy levels, surface, and structure. UV–vis and PL are sensitive probes of electronic transitions and electronic structures while IR and Raman are sensitive to vibration or phonon structures. Dynamic light scattering is useful for determining global structure of nanomaterials. Time-resolved techniques are used to determine dynamic properties such as charge carrier lifetimes [134].

Besides optical spectroscopy, a number of other experimental techniques are routinely used for characterizing nanomaterials, including electron microscopy, electrochemistry, X-ray-based methods such as X-ray diffraction (XRD) [13], [45], [135], [136], XPS, and EXAFS. While microscopy techniques are usually used to reveal structural, morphological, and topological information, X-ray techniques are applied to gain information about structure, energetic, and surface characteristics [1], [128], [137], [138], [139], [140], [141], [142], [143], [144], [145]. X-ray based spectroscopies are useful in determining the chemical composition of materials. These techniques include X-ray absorption spectroscopy (XAS), extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), X-ray fluorescence spectroscopy (XRF), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) [146], [147]. They are mostly based on detecting and analyzing radiation absorbed or emitted from a sample after excitation with X-rays, with the exception that electrons are analyzed in XPS. The spectroscopic features are characteristic of specific elements and thereby can be used for sample elemental analysis. Other related techniques include small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) that are useful for structural determination [148], [149], [150]. SAXS arises from inhomogeneous electron density on different length scales ranging from angstroms to microns, while WAXS primarily arises from regular, periodic variations of electron density over length scales that are large compared to the repeat distance.

Scanning probe microscopy (SPM) represents a group of techniques, including scanning tunneling microscopy (STM), atomic force microscopy (AFM), and chemical force microscopy (CFM), that have been extensively applied to characterize nanostructures [137], [143], [144], [151]. A common characteristic of these techniques is that an atomically sharp tip scans across the specimen surface and the images are formed by either measuring the current flowing through the tip or the force acting on the tip. SPM can be operated in a variety of environmental conditions and in different liquids or gases, allowing direct imaging of inorganic surfaces as well as organic molecules. It also allows manipulation of objects on the nanoscale. For non-conductive nanomaterials, AFM is a better choice [143], [144], [145]. AFM operates in an analogous manner as STM except the signal is the force between the tip and the solid surface. The interaction between two atoms is repulsive at short-range and attractive at long-range. The force acting on the tip reflects the distance from the tip atom(s) to the surface atom, thus images can be formed by detecting the force while the tip is scanned across the specimen.

Scanning electron microscopy (SEM) is a powerful and popular technique for imaging the surfaces of almost any material with a resolution down to about 1 nm [138], [139]. The image resolution offered by SEM depends not only on the property of the electron probe, but also on the interaction of the electron probe with the specimen. Interaction of an incident electron beam with the specimen produces secondary electrons along with back-scattered electrons. While the secondary electrons detected in SEM provide primarily topographic information, e.g. surface texture and roughness, back-scattered electrons afford both topographic and compositional information [152], [153]. Similarly, transmission electron microscopy (TEM) is a high spatial resolution structural and chemical characterization tool [154]. A modern high resolution TEM (HRTEM) has the capability to directly image atoms in crystalline specimens at resolutions close to 1 Å, smaller than the interatomic distance. This type of analysis is extremely important for characterizing materials at a length scale from atoms to hundreds of nanometers. TEM can be used to characterize nanomaterials to gain information about particle size, shape, crystallinity, and interparticle interaction [138], [155].

Section snippets

Optical properties of HSNs

For convenience of discussion, we will divide the HSNs into different groups based on the chemical nature of their components. We will be concerned primarily with two-component systems, denoted as AB, with one component (A) being a primary inorganic semiconductor. The other, secondary component (B) can be inorganic, organic, or biological. The following discussion will follow the classification of A being the primary semiconductor and B being the secondary component that can be an insulator

Optical functionalities and applications of HSNs

The optical properties and associated functionalities of hybrid nanostructures have many important applications in various technologies. We will review some examples of applications including energy conversion, sensing, photonics, environmental protection, and biomedical detection, imaging, and therapy.

Summary

A number of hybrid semiconductor nanomaterial (HSN) systems have been discussed with their optical properties highlighted. Besides the systems presented, there are many other hybrid nanostructures that we did not cover, including inorganic metal–metal, that tend to have strong interaction between their constituent components and thereby complex optical properties. The examples covered illustrate the diversity and usefulness of hybrid materials. There are practically unlimited possibilities for

Acknowledgements

We are grateful to financial support by the Basic Energy Sciences Division of the US DOE (DE-FG02-05ER46232-A002 and DE-FG02-07ER46388-A002), the US NSF (ECCS-0823921), National Natural Science Foundation of China (No. 20675044, No. 20628303) and National Basic Research Program of China (No. 2007CB310500).

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