N-type Nanocrystals Research Articles

A General Route for Chemiluminescence of n-Type Au Nanocrystals.

The photoluminescence, electroluminescence, and electrochemiluminescence from nanocrystals (NCs) have been extensively exploited for both fundamental and applied investigation over two decades, while the understanding of chemiluminescence (CL) from NCs is still far from clear by now. Herein, a general route for triggering CL from NC luminophore is proposed by extensively exploiting the charge transfer between n-type NCs and oxidants. Oxidants, such as K2S2O8, H2O2, KMnO4, and NaClO, can chemically inject the hole onto the valence band (VB) of methionine-capped n-type AuNCs (Met@AuNCs) and enable the occurrence of efficient radiative-charge-recombination between the chemically injected exogenous VB hole and the pre-existed endogenous conduction band (CB) electron, which eventually results in single-color and defect-involved CL with the maximum emission wavelength around 824 nm. The CL of Met@AuNCs/oxidant is qualified for ultrasensitive CL immunoassay in a similar procedure to the biotin-avidin and magnetic separation involved commercial CL immunoassay and exhibits acceptable performance for linearly determining carcinoembryonic antigen from 50 pg/mL to 100 ng/mL with a limit of detection of 10 pg/mL (S/N = 3). This strategy provides a general route to develop nanoparticulate CL luminophores and might eventually enable CL multiplexing assay via extensively exploiting the CL of different wavebands.

Coreactant-Free and Direct Electrochemiluminescence from Dual-Stabilizer-Capped InP/ZnS Nanocrystals: A New Route Involving n-Type Luminophore.

Electrochemiluminescence (ECL) is conventionally generated in either an annihilation or a coreactant route, and the overwhelming majority of ECL research is conducted in the coreactant route via oxidizing or reducing the coexisting coreactant and luminophore. The coreacant-free ECL generated via merely oxidizing the luminophore would break through the ceiling of coreactant ECL via excluding the detrimental effects of exogenous coreactant and dissolved oxygen. Herein, by exploiting the rich-electron nature of n-type nanocrystals (NCs), coreacant-free ECL is achieved via merely oxidizing 3-mercaptopropionic acid (MPA) and mercaptosuccinic acid (MSA) capped InP/ZnS NCs, i.e., InP/ZnSMPA-MSA. The electron-rich InP/ZnSMPA-MSA can be electrochemically injected with holes via two oxidative processes at around +0.75 and +1.37 V (vs Ag/AgCl), respectively, and the exogenous hole can directly combine the conduction band (CB) electron of InP/ZnSMPA-MSA, resulting in two coreactant-free ECL processes without employing any exogenous coreactant. The deprotonation process for the carboxyl group of the capping agents can provide a negatively charged surface to InP/ZnSMPA-MSA and enhance the coreactant-free ECL. The hole-injecting process at +1.37 is much stronger than that at +0.75 V and eventually enables an ∼2000-fold enhanced ECL at +1.37 V than that at +0.75 V. The ECL at +1.37 V can be utilized for coreactant-free ECL immunoassay with prostate-specific antigen (PSA) as analyte, which exhibits an acceptable linear response from 5 pg·mL-1 to 1 ng·mL-1 with a limit of detection of 0.3 pg·mL-1. The coreactant-free ECL route would provide an alternative to both annihilation and coreactant routes, simplify the ECL assay procedure and deepening the ECL mechanism investigations.

Plasmonic effect in pn-junction solar cells based on layers of semiconductor nanocrystals: Where to introduce metal nanoparticles?

We have introduced metal nanoparticles in pn-junction solar cells to study plasmonic effect in such devices. Here, the junction was based on a layer of copper–zinc–tin–sulfide (CZTS) nanocrystals as a p-type semiconductor and another layer of copper-diffused silver indium disulfide (Cu@AgInS2) nanocrystals as an n-type material in sequence. We introduced silver nanoparticles at different locations of pn- and also of np-junctions: (i) in the p-layer, (ii) in the n-layer, (iii) in both the layers, and (iv) at the interface between the layers of p- and n-type nanocrystals. The results in both pn- and np-junctions show that the devices with metal nanoparticles in p-type layer evidenced a substantial increase in energy conversion efficiency of solar cells as compared to other devices and the control device without any silver nanoparticles. We have inferred that the presence of silver nanoparticles in the depletion layer might have narrowed down the width of the region; metal nanoparticles in the n-layer acted as electron-traps to localize them hindering electron-transport and thereby reducing the efficiency of such plasmonic solar cells. Our results have shown that while introducing metal nanoparticles in solar cells, they should be placed only in the p-layer so that transport of electrons in plasmonic solar cells remains unaffected.

Aligned Magnetic Domains in p- and n-Type Ferromagnetic Nanocrystals and in pn-Junction Nanodiodes

We form pn- and np-junctions between monolayers of p- and n-type nanocrystals that exhibit current rectification in the nanodiodes when characterized with a scanning tunneling microscope (STM) tip. With the use of ferromagnetic nanocrystals, we study the effect of mutual alignment of magnetization vectors on current rectification in the junction between the two nanocrystals. We show that when the magnetization vectors of the p- and of the n-type nanocrystals are parallel to each other (and both facing toward the apex of the STM tip) tunneling current in both bias modes increases with correspondingly a higher rectification ratio. This is in contrast to the parameters of the nanodiodes in which magnetization vectors of the components are unaligned or randomized. To analyze the results, we record scanning tunneling spectroscopy of the monolayer of the components having magnetization vectors aligned or unaligned to locate their valence and conduction band edges and to determine the effect of the alignment on the band edges. Upon alignment of the magnetization vectors of the nanocrystals in a monolayer, the conduction band edge of the p-type and valence band edge of the n-type semiconductor shift towards the Fermi energy leading to a change in energy levels of the pn-junctions and accounting for the improved parameters of the nanodiodes.

Electronic Properties of Hybrid Materials Consisting of N-type Nanocrystals and Amorphous Selenium Matrix.

Thermal and electrical properties in hybrid materials consisting of p-type amorphous selenium and n-type In2Se3 nanocrystals have been investigated. The In-Se system shows a monotectic reaction at the selenium rich region in the solidification process. A rapid quenching method was therefore used in order to fabricate amorphous samples embedding nanocrystals of which the average size was 45 nm under the present condition.In the hybrid samples, melting point depression is observed by DTA measurements, and increases with increasing the numbers of the nanocrystals for an effect of the interface between the nanocrystals and the amorphous Se matrix. Green bodies of the amorphous Se are found to show negative differential conductivity (NDC). It seems that NDC is caused from dangling bonds, whose numbers are determined in the amorphization process. The dependence of the electrical conductivity of the hybrid green bodies on the In2Se3 concentration changes rapidly due to percolation transition. Electrical modification of amorphous Se can be performed by the addition of n-type nanocrystals, In2Se3. It is important for material design to modify the electrical properties without any restriction, because the electrical modification of an amorphous matrix by adding impurities has a solubility limit. The electrical properties in the present hybridisation are described in terms of the interfacial effect and the percolation phenomenon.

Organic-capped ZnO nanocrystals: synthesis and n-type character.

Wurtzite ZnO nanocrystals capped with trioctylphosphine oxide or alkylamines are synthesized and characterized. These ZnO nanocrystals can be made n-type either by electron transfer doping from reducing species in solution or by above band gap photoexcitation with a UV lamp. The n-type nanocrystals exhibit a strong intraband infrared absorption, an extensive bleach of the interband band-edge absorption, and a complete quenching of the photoluminescence.

A ‘channel’ design using single, semiconductor nanocrystals for efficient (opto)electronic devices

An architecture in which n- and p-type materials alternate laterally in the active film of a device with a spatial period of about a carrier diffusion length would lead to efficient (opto)electronic devices for photovoltaic energy conversion and switching, among others. Such an architecture, although difficult to achieve with conventional silicon-based schemes, is within reach using semiconductor nanocrystals, whose intrinsic size can be varied on a nanometer scale. A photovoltaic device whose active film contains single semiconductor n-type nanocrystals embedded in a conjugated polymer p-type phase would be one realization of the channel architecture. An npn transistor, where a single, semiconductor nanocrystal serves as emitter and collector (n-type) separated by p-type material, could exhibit significantly higher switching speeds than is possible with conventional devices. The channel architecture is particularly useful for devices which rely on low-mobility materials.

N-type colloidal semiconductor nanocrystals.

Colloidal semiconductor nanocrystals combine the physical and chemical properties of molecules with the optoelectronic properties of semiconductors. Their colour is highly controllable, a direct consequence of quantum confinement on the electronic states. Such nanocrystals are a form of 'artificial atoms' (ref. 4) that may find applications in optoelectronic systems such as light-emitting diodes and photovoltaic cells, or as components of future nanoelectronic devices. The ability to control the electron occupation (especially in n-type or p-type nanocrystals) is important for tailoring the electrical and optical properties, and should lead to a wider range of practical devices. But conventional doping by introducing impurity atoms has been unsuccessful so far: impurities tend to be expelled from the small crystalline cores (as observed for magnetic impurities), and thermal ionization of the impurities (which provides free carriers) is hindered by strong confinement. Here we report the fabrication of n-type nanocrystals using an electron transfer approach commonly employed in the field of conducting organic polymers. We find that semiconductor nanocrystals prepared as colloids can be made n-type, with electrons in quantum confined states.