Non-invasive precise thermometers working at the nanoscale with high spatial resolution, where the conventional methods are ineffective, have emerged over the last couple of years as a very active field of research. This has been strongly stimulated by the numerous challenging requests arising from nanotechnology and biomedicine. This critical review offers a general overview of recent examples of luminescent and non-luminescent thermometers working at nanometric scale. Luminescent thermometers encompass organic dyes, QDs and Ln3+ions as thermal probes, as well as more complex thermometric systems formed by polymer and organic–inorganic hybrid matrices encapsulating these emitting centres. Non-luminescent thermometers comprise of scanning thermal microscopy, nanolithography thermometry, carbon nanotube thermometry and biomaterials thermometry. Emphasis has been put on ratiometric examples reporting spatial resolution lower than 1 micron, as, for instance, intracellular thermometers based on organic dyes, thermoresponsive polymers, mesoporous silica NPs, QDs, and Ln3+-based up-converting NPs and β-diketonate complexes. Finally, we discuss the challenges and opportunities in the development for highly sensitive ratiometric thermometers operating at the physiological temperature range with submicron spatial resolution.

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Thermometry at the nanoscale

ZnO nanostructures for optoelectronics: Material properties and device applications

Photocatalytic degradation of organic dyes with manganese-doped ZnO nanoparticles.

Transparent conducting oxides for electrode applications in light emitting and absorbing devices

Ongoing advances in nanotechnology research have established a variety of methods to synthesize nanoparticles (NPs) from a diverse range of materials, including metals, semiconductors, ceramics, metal oxides, polymers, etc. Depending upon their origin and synthesis methods, NPs possess unique physicochemical, structural and morphological characteristics, which are important in a wide variety of applications concomitant to electronic, optoelectronic, optical, electrochemical, environment and biomedical fields. This review provides a comprehensive overview on various physical, chemical and bio-assisted methods largely employed to synthesize and fabricate NPs of varying size, surface characteristics, functionalities and physicochemical behavior. The key applications of nanoparticles have also been discussed.

Methods and strategies for the synthesis of diverse nanoparticles and their applications: a comprehensive overview

Thermoelectric properties and electrical characteristics of sputter-deposited p-CuAlO2 thin films

Al doped SnO2 thin films have been synthesized by a sol-gel dip coating technique with different percentages of Al on glass and silicon substrates. X-ray diffraction studies confirmed the proper phase formation in the films and atomic percentage of aluminium doping in the films was obtained by energy dispersive X-ray studies. SEM studies showed the particle sizes lying in the range 100–150 nm for the undoped films and it decreased with increase of Al doping. Optical transmittance spectra of the films showed high transparency (∼80%) in the visible region and the transparency increases with the increase of Al doping in the films. The direct allowed bandgap of the films have been measured for different Al concentration and they lie within the range of 3.87–4.21 eV. FTIR studies depicted the presence of Sn–O, Al–O, bonding within the films. The room temperature electrical conductivities of the films are obtained in the range of 0.21 S cm−1 to 1.36 S cm−1 for variation of Al doping in the films 2.31–18.56%. Room temperature Seebeck coefficients, SRT of the films were found in the range +56.0 μVK−1 to −23.3 μVK−1 for variation of Al doping in the films 18.56–8.16%. It is observed that the Seebeck coefficient changes its sign at 12.05% of Al in the films indicating that below 12.05% of Al doping, SnO2:Al behaves as an n-type material and above this percentage it is a p-type material.

Effect of Al doping on the conductivity type inversion and electro-optical properties of SnO2 thin films synthesized by sol-gel technique

Zn1−xMnxO sub-micron fibrous thin films were synthesized by a simple sol–gel dip-coating technique. XRD pattern confirmed the hexagonal wurtzite structure of the synthesized material. Expansion of the c-axis lattice parameter has been observed with the increase of manganese content in the films. Manganese content (x) in the films varied from 0.0098–0.0568 and was determined by energy-dispersive X-ray analysis. The average diameter of the ZnO fibers were ∼700 nm whereas the length was several hundred micro-meters and hence the aspect ratio was quite large (>150). Branching and uneven growth occurred with the increase of manganese content as is evident from SEM analysis. Optical transmittance spectra showed absorption band due to characteristic d–d transition of Mn2+ ions nearly at 3 eV. The band gap, as determined from the transmittance spectra, decreased to 3.28 eV for x = 0.0287 and then increased upto 3.38 eV for x = 0.0568 as compared with the undoped film for which the band gap is 3.31 eV. Photoluminescence spectra of Zn1−xMnxO fibrous thin films on glass substrate for different doping levels were also studied which showed a broad UV band at ∼397 nm, a narrow blue–green band at ∼466 nm and another broad peak at ∼526 nm. Mn doping causes a large reduction in the intensity of the UV emission and small reduction in green emission intensity of the PL spectra.

P.K. Ghosh

Papers

Thermoelectric properties and electrical characteristics of sputter-deposited p-CuAlO2 thin films

Effect of Al doping on the conductivity type inversion and electro-optical properties of SnO2 thin films synthesized by sol-gel technique

Effect of Mn doping on the optical and structural properties of ZnO nano/micro-fibrous thin film synthesized by sol-gel technique

Electrical and optical properties of highly conducting CdO:F thin film deposited by sol–gel dip coating technique

Photoluminescence and field emission properties of ZnS:Mn nanoparticles synthesized by rf-magnetron sputtering technique