Academic literature on the topic 'I-III-VI2 Nanocrystals'

Ternary I-III-VI2 nanocrystals (NCs), such as AgInS2 and CuInS2, are garnering interest as heavy-metal-free materials for photovoltaics, luminescent solar concentrators, LEDs, and bioimaging. The origin of the emission and absorption properties in this class of NCs is still a subject of debate. Recent theoretical and experimental studies revealed that the characteristic Stokes-shifted and long-lived luminescence of stoichiometric CuInS2 NCs arises from the detailed structure of the valence band featuring two sublevels with different parity. The same valence band substructure is predicted to occur in AgInS2 NCs, yet no experimental confirmation is available to date. Here, we use complementary spectroscopic, spectro-electrochemical, and magneto-optical investigations as a function of temperature to investigate the band structure and the excitonic recombination mechanisms in stoichiometric AgInS2 NCs. Transient transmission measurements reveal the signatures of two subbands with opposite parity, and photoluminescence studies at cryogenic temperatures evidence a dark state emission due to enhanced exchange interaction, consistent with the behavior of stoichiometric CuInS2 NCs. Lowering the temperature as well as applying reducing electrochemical potentials further suppress electron trapping, which represents the main nonradiative channel for exciton decay, leading to nearly 100% emission efficiency.

CuInSe2 I–III–VI2 ternary semiconductor considered as one of the most promising semiconductor material which considers a very efficient solar energy conversion material. An organometallic pyrolysis method is used to prepare monodisperse CuInSe2 nanoparticles using a mixture of oleylamine, and trioctylphosphine (TOP) as capping materials. Controlling the particle shape dot, rods or flowers occurs via varying the reaction temperatures (160, 200, 220°C) respectively. The obtained particles have been characterized to determine the shape and size of CuInSe2 nanoparticles using HR-TEM and XRD. The optical and the electronic properties of these particles have been investigated and discussed in details. Then the different shapes of CIS nanoparticles (nanodots, nanorods, and nanoflowers) were introduced to the DSSC to study their effect on the optical switching properties. It was found that the nanoflowers provide better photovoltaic performance than the other shapes; since it reduces the settling time to 50 milliseconds after it was more than 17 second before adding CIS nanoparticles to the cells.

As heavy-metal-free alternatives to Cd- and Pb-containing semiconductor nanocrystals (NCs), ternary I–III–VI2 compounds NCs have been actively studied, especially the CuInS2 NCs. Recently, it has been found that thick ZnS shelling can greatly improve the photochemical stability of such NCs, which undoubtedly enhances their application potential although it is still limited by the development of synthetic methods. This paper provides a facile method for preparation of thick-shell CuInS2/ZnS NCs. The effects of reaction temperatures ([Formula: see text]C and [Formula: see text]C) and sulfur precursors (dodecanethiol and sulfur powder) on the shell overgrowth are discussed in detail. When low reaction temperature ([Formula: see text]C) and inactive sulfur precursor are used, the overgrowth of ZnS shell is considerably slow and raising temperature have a limited impact on the particles’ size. On the contrary, high reaction temperature and reactive sulfur precursor can effectively improve the overgrowth rate of ZnS shell, and then thick-shell CuInS2/ZnS NCs can be received. Furthermore, a high-speed centrifugation method is used to screen out product NCs with a relatively uniform size.

I-III-VI2 ternary semiconductor nanocrystals, such as CuInS2 and AgInS2, exhibiting the quantum size effect have attracted much attention for the application to solar energy conversion systems because of their strong absorption coefficient and low toxicity. The optical properties of these particles are tunable by controlling the particle size. Recently we have successfully prepared anisotropic-shaped nanocrystals of ZnS-AgInS2 solid solution ((AgIn)xZn2(1-x)S2, ZAIS). Their photocatalytic H2 evolution activity could be controlled by the chemical composition as well as by the particle size,(1) and increased with particle morphology in the order of rice < sphere < rod.(2) On the other hand, the formation of type II heterojunction between different semiconductors was reported to be another strategy to enhance the photocatalytic activity of composite particles due to the effective charge separation of photogenerated electrons and holes at the heterojunction. In this study, we synthesize dumbbell-shaped nanocrystals composed of ZnS-AgInS2 solid solution (ZAIS) via epitaxial crystal growth in the solution phase, in which the heterojunction forms between rod- and rice-shaped parts in the nanocrystals. The photocatalytic activity of resulting nanocrystals is investigated for H2 evolution as a model reaction. Rod-shaped ZAIS nanocrystals with sizes of 4.1 × 23 nm as a precursor were prepared by the previously reported method.(2) These nanocrystals were heat-treated at 170 °C for 8 min in an oleylamine/1-dodecanethiol mixture solution containing AgCH3COO, In(CH3COO)3, and thiourea. Thus-obtained mixture nanocrystals were isolated from the resulting solution by adding methanol as a non-solvent. Dumbbell-shaped nanocrystals were separated from rice-shaped ones as a by-product with use of a size-selective precipitation technique. The photocatalytic activity for H2 evolution was investigated by the irradiation of dumbbell- and rice-shaped ZAIS nanocrystals in a mixture solution of water/2-propanol (1:1) containing Na2S as a hole scavenger with a Xe lamp (λ > 350 nm). The XRD analysis revealed that these ZAIS particles had a wurtzite crystal structure. With TEM measurements, we found that rice-shaped crystals with sizes of 5.6 × 11 nm were epitaxially grown on both the tips of rod-shaped ZAIS nanocrystals, resulting in the formation of dumbbell-shaped nanocrystals. The energy gap of dumbbell-shaped nanocrystals was determined to 1.9 eV from the absorption onset, being equal to that of freely dispersed nanocrystals with rice shape but lower than that of original rod-shaped ones, 2.8 eV. These suggested that the Zn content in rod-shaped parts of dumbbell-shaped nanocrystals was higher than that in rice-shaped parts of the same particles. By estimating the electronic energy structure of dumbbell-shaped ZAIS nanocrystals from those of corresponding rod- and rice-shaped nanocrystals, the heterojunction of type II structure was expected to form at the interface between rod- and rice-shaped parts in a dumbbell nanocrystal. ZAIS nanocrystals were dispersed in water/2-propanol solution and irradiated with a Xe lamp light. The H2 evolution was observed, the amount of which increased linearly with elapse of irradiation time. The H2 evolution rate of dumbbell nanocrystals was about four times larger than that of rice-shaped ones. These results indicated that the ZAIS nanocrystals worked as a photocatalyst and then the type II heterojunction in dumbbell-shaped nanocrystals induced the effective charge separation of photogenerated electrons and holes. Reference (1) T. Torimoto et al., J. Phys. Chem. C., 2015, 119, 24740-24749. (2) T. Torimoto et al., ACS Appl. Mater. Interfaces 2016 , 8, 27151-27161.

This thesis, entitled “A Study of Photophysics and Photochemistry of I-III-VI2 Nanocrystals” primarily deals with the properties of I-III-VI2 semiconductor nanocrystals composed of earth abundant, environmentally benign and relatively non-hazardous elements. In initial two chapters, the synthesis and photophysics of CuFeS2 and CuAlS2 QDs have been described. Both materials are potential candidates for various optoelectronic applications, and this makes the study of their physical properties interesting and relevant. Chapter 5 shows the light harvesting potential of I-III-VI2 QDs by using these to perform efficient artificial photosynthesis. Chapter two describes the stable synthesis and interesting optical properties of CuFeS2 and its core shell structures. These materials exhibit a tunable band gap that spans the range of 0.5 – 2 eV (600 nm – 2500 nm). Although the as-prepared material is non-emissive, CuFeS2/CdS core/shell structures are shown to exhibit quantum yields that exceed 80%. Like other members of the I-III-VI2 family QDs, CuFeS2 based nanoparticles exhibit a long- lived emission that is significantly red shifted compared to the band gap. Chapter three shows the various optical properties of CuAlS2 based QDs through calculation and ultrafast studies. CuAlS2/CdS QDs are shown to be associated with cross sections lower than 10-17 cm2 under the emission band. Investigation of this anomaly using spectroscopic techniques are described, and further, it is ascribed to the existence of a strong type-II offset between CuAlS2 and CdS layers. Besides their strong Stokes’ shift, CuAlS2/CdS QDs also exhibit high quantum yields (63%) as well as long emission lifetimes (~1500 ns). Finally the construction of a wide area transparent lighting device with a clear aperture of 7.5 cm2 is discussed. In Chapter four, the physical reason behind the stability of these I-III-VI2 QDs has been investigated. The optical properties of copper containing II-VI alloy quantum dots (CuxZn¬yCd1-x-ySe) were studied. Copper mole fractions within the host are varied from 0.001 to 0.35. No impurity phases are observed over this composition range. The optical absorption and emission spectra of these materials are observed to be a strong function of copper mole fractions, and provide information regarding composition induced impurity-impurity interactions. In particular, the integrated cross section of optical absorption per copper atom changes sharply with mole fraction of copper around 12%, suggesting a composition induced change in local electronic structure. In chapter five as photo reductive solar energy harvesters, it is shown that newly synthesized CuAlS2/ZnS QDs offer unprecedented advantages: these are composed of completely biocompatible, earth abundant, inexpensive elements; these exhibit very high solar to chemical energy conversion efficiencies and finally, light harvesting via these materials may be set up to reduce the carbon dioxide already present within the earth’s atmosphere. CuAlS2/ZnS structures can reduce aqueous bicarbonate ions to formate under visible light. The high turnover numbers (>7x104 molecules of sodium formate produced per QD), solar to chemical energy conversion efficiencies (20.2 +/- 0.2) are rationalized through our spectroscopic studies that show a short 550 fs electron dwell times in these structures. The high energy efficiency and the environmentally friendly composition of these materials suggest a future role in solar light harvesting.
DST, IISc, ISRO

Nanomaterials have been a topic of extensive research for the past several decades. This is because their properties act as a bridge between their bulk and atomic counterparts. Broadly, nanoparticles can be characterized as one dimensional, 2 dimensional or 3 dimensional depending on the number of directions in which particle size is limited. A semiconductor nanoparticle which is dimensionally limited in all the three directions is known as a Quantum Dot. In Quantum dots (QDs) the photoexcited charge carriers are constrained in a small volume and are not free to move in any direction. This causes an increased overlap between the electron and hole wavefunctions. As a result quantum dots possess many fascinating properties which render them useful in the field of optoelectronics, photovoltaics, and so on. However the use of QDs as efficient photocatalysts is not known. Recently, CuAlS2/ZnS QDs were reported which could reduce aqueous Sodium bicarbonate ions to formate ions using visible radiation. The average energy conversion efficiency obtained was 17% with a maximum of 20% with a turnover number of 7.8 x 104 that is significantly greater than any values reported previously. However, the major reported reduction product is sodium formate. From an energy perspective, it would be much more beneficial to have combustible organics as the reduction product as these could be directly used as fuels. Even though the CuAlS2/ZnS QDs could eventually reduce bicarbonate into organics like butanol, it takes weeks for the reaction to complete. It would hence be highly desirable to have a catalyst that could do this over a much shorter duration. In the second chapter of my thesis I have synthesized CuGaS2/ZnS nanoparticles. These quantum dots are capable of photo-reducing aqueous sodium bicarbonate into a mixture of alcohols using visible light. This is enormously advantageous. The material is made completely of biocompatible elements which makes processing and use of this material entirely safe for environment. The materials used are earth abundant and this reduces the manufacturing cost of the catalysts. The photo-reduced products (mainly butanol) can be used as a fuel and reducing bicarbonate can help reduce the global warming by decreasing CO2 levels in the atmosphere. Third chapter of my thesis elucidates the synthesis and optical properties of Copper Iron Aluminum Sulphide (CuFexAl1-xS2) QDs and its core shell structure with CdS. The band gap of CuAlS2 is 3.45 eV while the band gap of CuFeS2 system is 0.5 eV. The alloyed CuFexAl1-xS2 thus can have a tunable band gap from 3.45 to 0.5 eV. We demonstrate two compositions i.e CuFe0.1Al0.9S2 and CuFe0.35Al0.65S2 which exhibit band gaps of 1.6 eV and 0.7 eV respectively. These hybrid materials are not luminescent as such but coating a CdS layer on top of these materials makes them luminescent by eliminating surface traps. The CdS coated CuFexAl1-xS2 material QDs also exhibit tunable photoluminescence and tunable life time. The CuFexAl1-xS2/CdS system manifests the properties of both CuFeS2/CdS and CuAlS2/CdS i.e high Stokes shift and reasonably high Quantum yields. The potential of these materials for transparent display devices was verified.

Copper iron chalcogenides constitute a promising class of optoelectronic materials courtesy of their narrow bandgaps and earth abundant constitution. However, they are yet to receive the attention they deserve due to the lack of easy synthetic protocols and poorly understood material properties. Discordant narratives in the literature regarding their optoelectronic properties has also prevented them from being used for device-based applications. This thesis is aimed at rectifying a few of these issues. The objective of this thesis is to synthesize and study the properties of copper iron chalcogenide nanocrystals viz., CuFeS2 and CuFeSe2, and to explore their utility in the context of optoelectronic devices. Chapter 1 provides a brief introduction to the fundamental concepts related to the work described in this thesis. The chapter further discusses the scope and motivation behind the work carried out in this thesis. Chapter 2 describes our efforts to assign the nature of a feature in the optical absorption spectrum of CuFeS2 nanocrystals occurring at ~500 nm. Using a combination of steady-state and time-resolved optical spectroscopy as well as transport measurements we assign the feature to be a localized surface plasmon resonance and attribute the peculiar properties exhibited by CuFeS2 nanocrystals to this feature. Further, the transport measurements revealed that films of these nanocrystals can support a photoresponse. Chapter 3 describes the fabrication and characterization of a broadband photodetector based on CuFeS2 nanocrystals. Briefly, we fabricated heterojunctions of CuFeS2 nanocrystals with bulk n type silicon and demonstrated a broadband photoresponse from 460 nm-2200 nm with response time of the order of microseconds. The photodetector was further found to possess a photothermal response that is bolometric in nature, which allows the device to sense hot objects at room temperature. Chapter 4 describes our efforts to synthesize and study the optoelectronic properties of CuFeSe2 and CuFeSe2-CdS core-shell nanocrystals. We synthesized CuFeSe2 nanocrystals and studied their properties using structural, optical and electrical characterization techniques. The nanocrystals were found to have a very narrow bandgap of 0.11 eV and were also found to exhibit a plasmon resonance at ~410 nm. We further found that the films of these nanocrystals exhibited a photoresponse in the MIR, thus making them a promising candidate for infrared photodetection. We further synthesized highly luminescent CuFeSe2-CdS core-shell nanocrystals and found that the energetic position of their emission is greatly dependent on the sequence in which the shell growth precursors are added to the reaction mixture. Using optical and structural characterization techniques, we find that there are two different core-shell variants that result from the synthesis and their formation is determined by which one of the shell growth precursors is added to the reaction mixture first. The key difference between the two variants were found to be the presence of an interfacial CdSe layer which occurs whenever the cation precursor is added to the reaction mixture first. Chapter 5 describes the synthesis of CuFexGa1-xS2 nanocrystals, a hitherto unknown composition of nanocrystals. Using alloying as a strategy, we synthesized CuFexGa1-xS2 nanocrystals corresponding to different Fe:Ga ratios. The properties of the resulting nanocrystals were found to be greatly dependent on their composition.