Semiconductor nanoparticles of very small size, or quantum dots, exhibit fascinating physical properties, completely different from their bulk varieties, mostly because of the quantum confinement effect. Due to their modified band structure, they particularly show attractive optoelectronic characteristics. Carbon nanotubes are a class of nanomaterials, which also possess wonderful optoelectronic properties and can revolutionize modern semiconductor technology to a great extent. Carbon nanotube field-effect transistors (CNTFETs) can replace standard MOSFETs in an array of devices and can function in a more effective way. When these two optoelectronic components combine together in nanocomposites, one may get advanced optoelectronic devices for widespread application in sensors, solar cells, energy storage devices, light-emitting diodes, electrocatalysts, etc.
- quantum dots
- carbon nanotubes
Advanced research in Materials Science introduced a new direction to control the properties of materials with grain size at nanometer level (1–100 nm). At such reduced scale, materials exhibit fascinating physical properties completely different from their bulk counterparts. The properties of this new class of materials, called nanomaterials, can be tuned by changing their grain size and hence find a wide range of applications. Semiconductor nanoparticles are useful ingredients for the development of optoelectronic devices. This is due to their striking optical and optoelectronic properties, which strongly differ from those of the corresponding bulk materials. In principle, this is related to the existence of quantum confinement effects of excitons in the nanoparticles. Bandgaps are significantly changed with the size of the nanoparticles, thus modifying the mechanism of radiative recombination of the electrons and holes, which lead to large changes in the emission wavelengths of the excited nanocrystals. Thus, even Si-based nanoelectronic structures may become luminescent with visible light emission, leading to the vast possibilities of optoelectronics .
Due to quantum confinement effect, the continuous energy states split into discrete states, with effective blue-shift in the bandgap of the nanomaterials relative to that of bulk materials. Presence of defects such as vacancies, interstitial atoms, surface and grain boundaries in the semiconductor nanocrystals and also doping with appropriate dopants introduce discrete energy levels, which alter the luminescence property of such nanomaterials. An increase in energy bandgap with decrease in particle size leads to a blue-shift in the absorption spectrum. The influence of particle size is found not only in the absorption spectrum but also in the wavelength of the emitted photons. Carbon nanostructures and carbon nanotubes (CNTs), in particular, have such remarkable electronic and structural properties that they are used as active building blocks for a large variety of nanoscale devices. Precisely, the use of carbon nanotubes as active components in electronic and optoelectronic nanodevices has great potential. When nanostructures are synthesized with the hybridization of the above two promising optoelectronic components, a new set of functional nanocomposites appear with fascinating optoelectronic characteristics that may lead to the fabrication of advanced optoelectronic devices for a great variety of potential applications.
2. Quantum confinement in semiconductor nanoparticles
If the size of a nanoparticle becomes comparable to or smaller than the radius of the orbit of the electron-hole pair, then we have a quantum dot (QD) and the case of quantum confinement arises. Quantum confinement can be well explained by applying the laws of quantum mechanics. Quantum confinement is observed in the case of interaction of very small nanocrystals with radiation, when free electrons and holes are created. The hole and the electron form a hydrogen-like system called an ‘exciton.’ The radius of the exciton, called the ‘excitonic Bohr radius,’ may range from a fraction of a nanometer to a few nanometers. Quantum confinement occurs when at least one dimension of a nanocrystal is smaller than the diameter of the exciton. In this case, the absorption and emission of light are strongly particle size-dependent and the phenomenon can be explained by the mechanism of quantum confinement. The behavior of subatomic particles is understood by using Schrödinger equation. In a bulk semiconductor, the electrons of the conduction band and the holes of the valence band are free to move throughout the crystal and their motion can be described by the linear combination of plane waves with wavelength of the order of nanometers. Whenever the size of a semiconducting solid becomes comparable to these wavelengths, a free charge carrier confined in this structure will behave as a particle in a potential box. In such a case, the solutions of the Schrödinger equation are standing waves confined within the potential well, and the energies associated with two distinct wave functions are in general different and discontinuous. Thus, the energy of the particle exhibits a discrete energy level spectrum. Brus  modelled in a simple way the excited electronic states of semiconductor crystallites of sufficiently small size. He expressed the energy of the lowest excited state as:
In Eq. (1), the second term describes the increase in energy due to confinement effects, while the third term describes the decrease in energy due to Coulomb interaction between electrons and holes.
For a zero-dimensional nanostructure, such as a quantum dot, the density of states is given by:
2.1. Optoelectronic properties of QDs
In this section, some significant recent research investigations on the striking optoelectronic properties of semiconductor nanocrystals will be discussed. Liu et al.  reported a facile chemical synthesis of SnSe nanocrystals with tunable sizes, by using common alkylphosphine-Se precursors in combination with SnCl2. The optoelectronic properties of colloidal SnSe NCs were studied by fabricating a SnSe NC thin film between interdigitated gold electrodes ( Figure 2(A) ). The current-voltage (I-V) curves were measured in the dark and under AM 1.5 simulated solar illumination. The film showed a significant photoresponse ( Figure 2(B) ). The photoresponse of the SnSe nanocrystal thin-film to simulated solar light indicates that the materials have potential for use in solution-processed photovoltaic and optoelectronic devices.
Stokes et al.  reported a wet chemical synthesis of semiconductor quantum dots (SQDs), such as cadmium sulphide (CdS) or cadmium selenide (CdSe), etc. They stabilized nanocrystals and controlled their size for specific optical and electronic properties, by chemical capping using some organic capping agent, such as tri-n-octylphosphine oxide (TOPO). They further manipulated their properties by using a core-shell structure of SQDs. Core-shell structures are made by covering the surface of a SQD with a shell of another SQD, which has a greater bandgap than the core (e.g., ZnS over CdS or CdSe). Also as discussed by them, epitaxial quantum dot films (EQDs) of III–V compound semiconductors (InAs/GaAs) have been widely used in infrared photodetectors. Figure 3 shows the schematics of a typical epitaxial InAs/GaAs infrared photodetector.
3. Carbon nanotubes
The atomic structure of carbon nanotubes depends on tube chirality, which is characterized by the chiral vector
CNT is constructed by rolling up the graphene sheet such that the two end-points of the vector
For an armchair tube,
Electronic transitions between the energy bands of carbon nanotubes and their standard designations are illustrated in the diagram below for single-walled carbon nanotubes (SWNTs), synthesized in three different techniques, namely hi-pressure carbon monoxide-chemical vapour deposition (HiPCO-CVD), laser ablation, and arc discharge method. Their absorbance characteristics are shown along with following Niyogi et al.  ( Figure 5C ).
3.1. Optoelectronic properties of carbon nanotubes
A great characteristic of carbon nanotubes is their ability to withstand a very high current density, exceeding 109 A/cm2 [7–9]. For metallic nanotubes, the current-carrying capability may even rise up to a value of 25 μA, limited by Joule self-heating, and by the scattering of optical phonons. The saturation current limit for semiconducting nanotubes is controlled by the Schottky barrier electrical resistance at the nanotube-metal contact, which in turn depends on the diameter of the tube and the work function of the contact metal.
A promising material for fabricating cold cathodes for the next-generation high-performance flat-panel devices is carbon nanotubes (CNTs). Sridhar et al.  synthesized a forest of vertically aligned MWCNTs on Inconel 718 substrate and studied its field emission properties. Compared to similar CNT structures grown on silicon, the ones on Inconel substrate were found to possess lower switching fields (~1.5 V/μm), higher operating current (~100 mA/cm2), and higher amplification factor (~7300), and thus promised enormous potential for use as cold cathodes in microwave vacuum devices, etc. ( Figure 6 ).
As explained by Kinoshita , a carbon nanotube field-effect transistor (CNTFET) may be designed in a simple way, with the carbon nanotube providing the transport channel when contacted on each end by bulk metal, and a heavily doped Si substrate acts as the back gate, with SiO2 performing the function of the gate dielectric. Unlike a conventional MOSFET, a CNTFET operates as a Schottky barrier transistor where the gate controls the Schottky barrier and the injection of carriers. In a CNTFET, one can obtain an
Topinka et al.  studied carbon nanotube network field-effect transistors (CNTN-FETs). They investigated the microscopic transport mechanism of such devices and observed that in CNTN-FETS the voltage dropped abruptly at a point in the channel where the current was constricted to just one tube. They varied the semiconducting/metallic tube ratio and studied the effect of Schottky barriers on conductance of the channel.
4. Nanoparticle-carbon nanotube composites
When the external surfaces of carbon nanotubes are decorated with semiconductor nanoparticles or quantum dots (NPs/QDs), a new class of functional materials is produced with remarkable properties, which combine the unique characteristics of the individual components. Such nanostructures are known as CNT-NP (or CNT-QD) hybrids, or nanocomposites. These nanocomposite materials are potentially useful in a wide range of advanced applications, in the field of chemical sensors , biosensors , electrocatalysis , fuel cells , and nanoelectronics . Semiconductor NPs of very small size are known as quantum dots (QDs), as they exhibit interesting size-dependent optical and electronic properties, due to quantum confinement effect. As observed by Georgakilas et al.  CNTs hybridized with QDs of II–VI or III–V semiconductors are ideal optoelectronic materials and can be used in solar cells, light-emitting diodes, etc.
Therefore, varieties of semiconductor NPs, such as CdSe, CdS, CdTe, PbSe, ZnS, ZnO, SiO2, TiO2, and In2S3, have been bound to CNT surfaces and properties studied by several researchers.
The electronic interaction between CNTs and the externally attached nanocrystals (NCs) plays a crucial role in constructing optoelectronic devices [19, 20]. Hybridizing semiconductor NPs on CNT surfaces can modify their optical characteristics and luminescent property significantly and hence can find application in assembling photoelectrochemical cells , in tailoring light-emitting diodes , in organizing sensor systems, and in fabricating electrochromic devices .
4.1. Synthesis and characterization of the CNT-QD composites
There are several methods to decorate carbon nanotubes with nanoparticles. Chen and Lu  proposed an efficient gas-phase route of the electrostatic force directed assembly (ESFDA) process. The locally enhanced electric field near CNTs results in directed assembly of charged aerosol nanoparticles onto CNTs. They coated MWCNTs with a mixture of SnO2 and Ag nanocrystals by using this method.
However, the most widely exploited route for synthesis of semiconductor nanoparticle-coated carbon nanotubes is the wet chemical process. In this method, the size of the QDs can be tuned and the proportions of the two components of the composite can be adjusted to obtain the desired properties. Carboxyl groups on the surface of acid-treated CNTs are often used to attach amine-terminated or mercapto-terminated inorganic nanoparticles through amide bonds.
Li et al.  used polyamine- and amino-functionalized MWCNTs (MWCNT-NH2) templates to load CdTe quantum dots (QDs) and Fe3O4 magnetic nanoparticles by the electrostatic self-assembly approach, synthesizing MWCNT/CdTe and MWCNT/Fe3O4 nanohybrids ( Figure 9 ).
Robel et al.  investigated the single-walled carbon nanotube-CdS nanocomposites as light-harvesting assemblies and studied their photoinduced charge-transfer interactions, undertaking a simple solution-mixing synthesis of the samples as shown in Figure 10 .
Paul et al.  reported a simple chemical precipitation technique for the synthesis of a hybrid nanostructure of single-wall carbon nanotubes (SWCNT) and titania (TiO2) nanocrystals of average size 5 nm ( Figure 11 ). They prepared SWCNT/TiO2 nanohybrid structures by mixing purified SWCNT in TiCl3 solution and stirring the mixture in a magnetic stirrer. NH4OH solution was added in drops to attain pH neutrality and the mixture was further stirred for a few hours. Then, they centrifuged the colloidal solution at 9°C, washed it with deionized water and 2-propanol, and dried up at room temperature. They did structural and morphological characterization of the samples, and studied their optical and electrical properties.
In the absorbance spectra of the nanohybrid material, a blue-shift was observed by them, confirming the charge transfer between SWCNTs and titania nanoparticles. There was a considerable visible emission in the photoluminescence spectrum with the peak emission at 400 nm.
Figure 12 shows the Raman spectrum of pristine SWCNT and that of SWCNT/TiO2 hybrid. The peaks at 150, 408, and 650 cm−1 of the hybrid sample correspond to the photoelectronically active TiO2 anatase phase. There is an upshift by 12 cm−1 in the position of G-band indicating charge transfer to SWCNTs from TiO2 NCs.
Study of DC conductivity of pristine SWCNT showed a crossover from a semiconductor-like temperature dependence of conductivity to a metal-like one with the increase of temperature, transition temperature being 180 K ( Figure 13 ). When coated with TiO2 NPs, the whole material behaves as a semiconducting material and its conductivity decreases by several orders of magnitude. A broad luminescence in the visible region in the range of 325–500 nm was observed, which could be attributed to the charge transfer from the titania NCs to the SWCNTs. Thus, the hybridization of TiO2 nanoparticles with SWCNTs rendered an advanced functional material with improved photocatalytic response due to reduced recombination of photoelectrons. Also, this nanocomposite could find application as a useful optical material for sensor devices.
4.2. Optoelectronic applications of CNT-QD composites
Yang et al.  synthesized In2S3-carbon nanotube nanocomposites through a facile refluxing wet chemistry process, as shown in the migrograph ( Figure 14 ). The as-synthesized In2S3-CNT nanocomposites were used as selective and active visible-light driven photocatalysts toward hydrogenation of nitroaromatics to amines in water, with enhanced photocatalytic performance ( Figure 15 ).
Shi et al.  reported a simple process for the synthesis of a sandwich structure of SWCNT-CdSe hybrid film (thickness ~ 200 nm), in between two SWCNT films (each thickness ~ 36 nm). They tested it as an optoelectronic conversion device under the illumination of simulated solar light. They found that the device could generate photocurrent with high sensitivity and their observations are illustrated in Figure 16 .
Attachment of semiconductor nanoparticles or quantum dots on the surfaces of carbon nanotubes has opened up innumerable possibilities of their applications in a variety of advanced nanodevices in various areas of nanotechnology. Combining the remarkable optoelectronic properties of the two outstanding nanoscale components, a vast array of fascinating optoelectronic devices is being proposed by researchers every day all over the world. In our brief review, we have touched upon a few of such applications.
One of the authors (Rima Paul) is indebted to Dr. D. S. Kothari Foundation for supporting her through Dr. D. S. Kothari Post Doctoral Fellowship at Indian Institute of Science, Bangalore, India.
Gonzalez-Varona O, Perez-Rodrõguez A, Garrido B, Bonafos C, Lopez M, Morante JR, Montserrat J, Rodrõguez R. Ion beam synthesis of semiconductor nanoparticles for Si-based optoelectronic devices. Nuclear Instruments and Methods in Physics Research B. 2000; 161–163: 904–908.
Brus LE. Electron-electron and electron-hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state. The Journal of Chemical Physics. 1984; 80: 4403–4409. DOI: 10.1063/1.447218
Liu X, Li Y, Zhou B, Wang X, Cartwright AN, Swihart MT. Shape-controlled synthesis of SnE (E = S, Se) semiconductor nanocrystals for optoelectronics. Chemistry of Materials. 2014; 26: 3515–3521. DOI: 10.1021/cm501023w
Stokes EB, Stiff-Roberts AD, Dameron, CT. Quantum dots in semiconductor optoelectronic devices. The Electrochemical Society Interface Winter. 2006; 15(4).
Charlier JC, Blase X, Roche S. Electronic and transport properties of nanotubes. Reviews of Modern Physics. 2007; 79: 677–732. DOI: 10.1103/RevModPhys.79.677
Niyogi S, Hamon MA, Hu H, Zhao B, Bhowmik P, Sen R, Itkis ME, Haddon RC. Chemistry of single-walled carbon nanotubes. Accounts of Chemical Research. 2002; 35: 1105–1113. DOI: 10.1021/ar010155r
Collins PG, Arnold MS, Avouris P. Engineering carbon nanotubes and nanotube circuits using electrical breakdown. Science. 2001; 292(5517): 706–709. DOI: 10.1126/science.1058782
Wei Y, Xie C, Dean KA, Coll BF. Stability of carbon nanotubes under electric field studied by scanning electron microscopy. Applied Physics Letters. 2001; 79(27): 4527–4529. DOI: 10.1063/1.1429300
Yao Z, Kane CL, Dekker C, High-field electrical transport in single-wall carbon nanotubes. Physical Review Letters. 2000; 84(13): 2941–2944. DOI: 10.1103/PhysRevLett.84.2941
Sridhar S, Ge L, Tiwary CS, Hart AC, Ozden S, Kalaga K, Lei S, Sridhar SV, Sinha RK, Harsh H, Kordas K, Ajayan PM, Vajtai R. Enhanced field emission properties from CNT arrays synthesized on inconel superalloy. ACS Applied Materials & Interfaces. 2014; 6: 1986–1991. DOI: 10.1021/am405026y
Kinoshita M. Optoelectronics with Carbon Nanotubes, Ph.D. Dissertation, State University of New York at Stony Brook: 2011; pp. 15–17.
Topinka MA, Rowell MW, Goldhaber-Gordon D, McGehee MD, Hecht DS, Gruner G. Charge transport in interpenetrating networks of semiconducting and metallic carbon nanotubes. Nano Letters. 2009; 9(5): 1866–1871. DOI: 10.1021/nl803849e
Kong J, Chapline MG, Dai HJ. Functionalized carbon nanotubes for molecular hydrogen sensors. Advanced Materials. 2001; 13: 1384–1386. DOI: 10.1002/1521-4095(200109)13
Chen RJ, Bangsaruntip S, Drouvalakis KA, Kam NWS, Shim M, Li YM, Kim W, Utz PJ, Dai HJ. Noncovalent functionalization of carbon nanotubes for highly specific electronic biosensors. Proceedings of the National Academy of Sciences, 2003 USA. 2003; 100: 4984–4989. DOI: 10.1073pnas.0837064100
Luo J, Jones VW, Maye MM, Han L, Kariuki NN, Zhong CJ. Thermal activation of molecularly-wired gold nanoparticles on a substrate as catalyst. Journal of the American Chemical Society. 2002; 124: 13988–13989. DOI: 10.1021/ja028285y
Che GL, Lakshmi BB, Fisher ER, Martin CR. Carbon nanotubule membranes for electrochemical energy storage and production. Nature. 1998; 393: 346–349. DOI: 10.1038/30694
Hu JT, Min OY, Yang PD, Lieber CM. Controlled growth and electrical properties of heterojunctions of carbon nanotubes and silicon nanowires. Nature. 1999; 399: 48–51. DOI: 10.1038/19941
Georgakilas V, Gournis D, Tzitzios V, Pasquato L, Guldi DM, Prato M. Decorating carbon nanotubes with metal or semiconductor nanoparticles. Journal of Materials Chemistry. 2007; 17: 2679–2694. DOI: 10.1039/B700857K
Strano MS, Dyke CA, Usrey ML, Barone PW, Allen MJ, Shan HW, Kittrell C,. Hauge RH, Tour JM, Smalley RE. Electronic structure control of single-walled carbon nanotube functionalization. Science. 2003; 301(5639): 1519–1522. DOI: 10.1126/science.1087691
Cao J, Sun JZ, Heng H, Li HY, Chen HZ, Wang M. Carbon nanotube/CdS core–shell nanowires prepared by a simple room-temperature chemical reduction method. Advanced Materials. 2004; 16(1): 84–87. DOI: 10.1002/adma.200306100
Hickey SG, Riley DJ, Tull EJ. Photoelectrochemical studies of CdS nanoparticle modified electrodes: Absorption and photocurrent investigations. Journal of Physical Chemistry B. 2000; 104: 7623–7326. DOI: 10.1021/jp993858n
Tessler N, Medvedev V, Kazes M, Kan SH, Banin U. Efficient near-infrared polymer nanocrystal light-emitting diodes. Science. 2002; 295(5559): 1506–1508. DOI: 10.1126/science.1068153
Bechinger C, Ferrer S, Zaban A, Sprague J, Gregg BA. Photoelectrochromic windows and displays. Nature. 1996; 383: 608–610. DOI: 10.1038/383608a0
Chen J, Lu G. Controlled decoration of carbon nanotubes with nanoparticles. Nanotechnology. 2006; 17: 2891–2894. DOI: 10.1088/0957-4484/17/12/011
Banerjee S, Wong SS. Synthesis and characterization of carbon nanotube − nanocrystal heterostructures, Nano Letters. 2002; 2: 195–200. DOI: 10.1021/nl015651n
Li W, Gao C, Qian H, Ren J, Yan D. Multiamino-functionalized carbon nanotubes and their applications in loading quantum dots and magnetic nanoparticles. Journal of Materials Chemistry. 2006; 16: 1852–1859. DOI: 10.1039/B600190D
Robel I, Bunker BA, Kamat PV. Single-walled carbon nanotube–CdS nanocomposites as light-harvesting assemblies: photoinduced charge-transfer interactions. Advanced Materials. 2005; 17: 2458–2463. DOI: 10.1002/adma.200500418
Paul R, Kumbhakar P, Mitra AK. A facile chemical synthesis of a novel photo catalyst: SWCNT/titania nanocomposite. Advances in Nano Research. 2013; 1(2): 71–82. DOI: 10.12989/anr.2013.1.2.071
Yang MQ, Weng B, Xu Y-J. Synthesis of In2S3–CNT nanocomposites for selective reduction under visible light. Journal of Materials Chemistry A. 2014; 2: 1710–1720. DOI: 10.1039/C3TA14370H
Shi Z, Liu C, Shen Lv W, Shen H, Wang D, Chen L, Li LS, Jin J. Free-standing single-walled carbon nanotube–CdSe quantum dots hybrid ultrathin films for flexible optoelectronic conversion devices. Nanoscale. 2012; 4: 4515–4521. DOI: 10.1039/C2NR30920C