A pinboard by
Youngjin Jang

I am a postdoc in Technion, Israel, focusing on colloidal synthesis of semiconductor nanocrystals.


The important background of semiconductor nanocrystals, including narrow bandgap nanocrystals

Q. What is a semiconductor?

A semiconductor material has an electrical conductivity value between that of a conductor, such as copper, and that of an insulator, such as glass.

Q. Why is Nano important?

A bulk semiconductor material has constant properties regardless of its size, however, when the size of material decreases into nanoscale, its properties are dramatically changed, showing size-dependency.

Q. What is quantum dots?

The size of bulk semiconductor materials becomes nanoscale, they are called as semiconductor nanocrystals or quantum dots. They exhibit the unique optical and electrical properties which differ from those of bulk materials. When electricity or light is applied to semiconductor nanocrystals, they can emit light of specific wavelength (or frequency). The position of emission can be tuned by adjusting the size, shape, and material, leading to various applications such as solar cell, biological tagging, and light-emitting diodes. For example, larger semiconductor nanocrystals emit longer wavelength resulting in emission colors such as orange or red. Smaller quantum dots emit shorter wavelengths resulting in colors like blue and green.

Q. What is the issue in narrow bandgap semiconductor?

Nowadays, beside visible range of optical properties, the demand of infrared optical activity is increasing. Therefore, the controlled synthesis of semiconductor nanocrystals showing infrared optical activities is highly important scientific and technological interest. For the fabrication of infrared devices, the lead chalcogenides have been studied and shown good performance, however, they contain lead element with the high toxicity. Therefore, for a long-run practical application, the use of heavy metal compounds should be avoided. Finding alternative semiconductor materials with low toxicity is an extremely important for scientific research and technological applications.


Semiconductor nanocrystals: structure, properties, and band gap engineering.

Abstract: Semiconductor nanocrystals are tiny light-emitting particles on the nanometer scale. Researchers have studied these particles intensely and have developed them for broad applications in solar energy conversion, optoelectronic devices, molecular and cellular imaging, and ultrasensitive detection. A major feature of semiconductor nanocrystals is the quantum confinement effect, which leads to spatial enclosure of the electronic charge carriers within the nanocrystal. Because of this effect, researchers can use the size and shape of these "artificial atoms" to widely and precisely tune the energy of discrete electronic energy states and optical transitions. As a result, researchers can tune the light emission from these particles throughout the ultraviolet, visible, near-infrared, and mid-infrared spectral ranges. These particles also span the transition between small molecules and bulk crystals, instilling novel optical properties such as carrier multiplication, single-particle blinking, and spectral diffusion. In addition, semiconductor nanocrystals provide a versatile building block for developing complex nanostructures such as superlattices and multimodal agents for molecular imaging and targeted therapy. In this Account, we discuss recent advances in the understanding of the atomic structure and optical properties of semiconductor nanocrystals. We also discuss new strategies for band gap and electronic wave function engineering to control the location of charge carriers. New methodologies such as alloying, doping, strain-tuning, and band-edge warping will likely play key roles in the further development of these particles for optoelectronic and biomedical applications.

Pub.: 16 Oct '09, Pinned: 30 Jul '17

Narrow bandgap colloidal metal chalcogenide quantum dots: synthetic methods, heterostructures, assemblies, electronic and infrared optical properties.

Abstract: The chemistry, material processing and fundamental understanding of colloidal semiconductor nanocrystals (quantum dots) are advancing at an astounding rate, bringing the prospects of widespread commercialization of these novel and exciting materials ever closer. Interest in narrow bandgap nanocrystals in particular has intensified in recent years, and the results of research worldwide point to the realistic prospects of applications for these materials in solar cells, infrared optoelectronics (e.g. lasers, optical modulators, photodetectors and photoimaging devices), low cost/large format microelectronics, and in biological imaging and biosensor systems to name only some technologies. Improvements in fundamental understanding and material quality are built on a vast body of experience spread over many different methods of colloidal synthetic growth, each with their own strengths and weaknesses for different materials and sometimes with regard to particular applications. The nanocrystal growth expertise is matched by a rapidly expanding, and highly interdisciplinary, understanding of how best to assemble these materials into films or hybrid composites and thereby into useful devices, and again there are many different strategies that can be adopted. In this review we have attempted to survey and compare the recent work on colloidal synthesis, film and nanocrystal composite material fabrication, concentrating on narrow bandgap chalcogenide materials and some of their topical applications in the solar energy and biological fields. Since these applications are attracting rising interest across a wide range of disciplines, from the biological sciences, device engineering, and materials processing fields as well as the physics and synthetic chemistry communities, we have endeavoured to make the review of these narrow bandgap nanomaterials both comprehensive and accessible to newcomers to the area.

Pub.: 31 Jan '13, Pinned: 30 Jul '17

Interface control of electronic and optical properties in IV-VI and II-VI core/shell colloidal quantum dots: a review.

Abstract: Semiconductor colloidal quantum dots (CQDs) have attracted vast scientific and technological interest throughout the past three decades, due to the unique tuneability of their optoelectronic properties by variation of size and composition. However, the nanoscale size brings about a large surface-to-bulk volume ratio, where exterior surfaces have a pronounced influence on the chemical stability and on the physical properties of the semiconductor. Therefore, numerous approaches have been developed to gain efficient surface passivation, including a coverage by organic or inorganic molecular surfactants as well as the formation of core/shell heterostructures (a semiconductor core epitaxially covered by another semiconductor shell). This review focuses on special designs of core/shell heterostructures from the IV-VI and II-VI semiconductor compounds, and on synthetic approaches and characterization of the optical properties. Experimental observations revealed the formation of core/shell structures with type-I or quasi-type-II band alignment between the core and shell constituents. Theoretical calculations of the electronic band structures, which were also confirmed by experimental work, exposed surplus electronic tuning (beyond the radial diameter) with adaptation of the composition and control of the interface properties. The studies also considered strain effects that are created between two different semiconductors. It was disclosed experimentally and theoretically that the strain can be released via the formation of alloys at the core-shell interface. Overall, the core/shell and core/alloyed-shell heterostructures showed enhancement in luminescence quantum efficiency with respect to that of pure cores, extended lifetime, uniformity in size and in many cases good chemical sustainability under ambient conditions.

Pub.: 21 Dec '16, Pinned: 30 Jul '17