Effects of Surface Chemistry on Nonlinear Absorption, Scattering, and Refraction of PbSe and PbS Nanocrystals

Talapin D.V., Lee J., Kovalenko M.V., Shevchenko E.V.

Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

Asunskis D.J., Bolotin I.L., Haley J.E., Urbas A., Hanley L.

A series of lead sulfide (PbS) nanocrystals grown in oleic acid (OA), poly(1-decene) (PD), poly(1-butene) (PB), and poly(vinyl alcohol) (PVA) were synthesized. The relationship between nonlinear optical properties and surface properties of PbS nanocrystals were analyzed using 532 and 1064 nm nanosecond open-aperture Z-scan measurements and femtosecond pump−probe experiments with a 390 nm pump and a 450−700 nm probe. The results indicated a clear difference in nonlinear optical properties between the OA and PVA composites in comparison to PB and PD. The materials with strong covalent bonding between the nanocrystal surface and organic phase, PVA and OA, did not show nonlinear absorption behavior in Z-scan measurements at 532 or 1064 nm. The PB and PD composites did, however, show strong reverse saturable absorption in Z-scans recorded at these wavelengths. The transient absorption measurements further separated the PVA and OA composites from the PD and PB samples. The decays for each of the samples indicat...

Neo M.S., Venkatram N., Li G.S., Chin W.S., Wei J.

We report the size-dependent optical nonlinearities of PbS nanoparticles both in solution and as a composite film. Different sizes of PbS nanoparticles were synthesized by chemical methods, with the sizes controlled by varying reaction temperature and time. Transparent PbS/polystyrene thin films deposited on glass slides were successfully prepared by a simple imprinting-thermal cross-linking method. We found that smaller nanoparticles show higher nonlinear absorption and refraction of photoexcited free carriers, as well as larger optical Kerr nonlinearity. Our studies also revealed that nonlinear scattering is strongly dependent on size and plays a dominant role in the case of bigger nanoparticles in solution at higher excitation energy.

Fernée M.J., Jensen P., Rubinsztein-Dunlop H.

We report hysteretic photophysical behavior over a 110 K temperature range in freshly prepared colloidal suspensions of PbS/oleic acid nanocrystals. A bistable regime consisting of both low and high absorbance states of the PbS nanocrystals is observed between 215 and 240 K. The change in absorbance is significant (increasing by at least a factor of 5) and is shown to be entirely nonexcitonic in origin. The absorbance change is correlated with a thermal hysteresis in the photoluminescence quenching rate associated with a sudden switching of the quenching behavior upon cooling below 220 K. Most surprisingly, these effects are temporary, resulting in either the complete destruction of the smallest nanocrystals or stabilization of the larger nanocrystals after many weeks. We attribute this anomalous behavior to a structural phase change in the oleate surface ligands. This phenomenon thus illustrates the potentially large effect that surfaces can have on photophysical properties of PbS nanocrystals. Furthermore it opens up the possibility of surface engineering the photophysical properties of these materials for different applications such as photovoltaics.

Xiang W., Tang S., Zhang X., Liang X.

Abstract 5.74Na2O–21.38B2O3–72.88SiO2 glasses doped with semiconductor 1% wt PbS nanocrystals were fabricated via a sol–gel route. The formation of PbS nanocrystals in the glass matrix was proven by X-ray photoelectron spectroscopy (XPS). The HRTEM images showed that the PbS crystals existed as nanocrystals with a narrow distribution diameter at 3.5 nm. Furthermore, Fast Fourier transformation (FFT) demonstrated the existence of PbS with face-centered cubic (FCC) structure. Comparing with the bulk PbS, the absorption edge in the optical absorption spectra of the glass doped with PbS nanocrystals exhibited a blue shift with an obvious quantum size effect. The glass had a nonlinear refractive index (γ) of −2.03 × 10−14 cm2/GW and a nonlinear absorption coefficient (β) of 7.05 × 10−10 cm/GW) based on Z-scan technique measurement.

Beard M.C., Midgett A.G., Law M., Semonin O.E., Ellingson R.J., Nozik A.J.

We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (NC) films. We find that the average number of excitons produced per absorbed photon varies between 1.0 and 2.4 (+/-0.2) at a photon energy of approximately 4E(g) for films consisting of 3.7 nm NCs and between 1.1 and 1.6 (+/-0.1) at hnu approximately 5E(g) for films consisting of 7.4 nm NCs. The variations in MEG depend upon the chemical treatment used to electronically couple the NCs in each film. The single and multiexciton lifetimes also change with the chemical treatment: biexciton lifetimes increase with stronger inter-NC electronic coupling and exciton delocalization, while single exciton lifetimes decrease after most treatments relative to the same NCs in solution. Single exciton lifetimes are particularly affected by surface treatments that dope the films n-type, which we tentatively attribute to an Auger recombination process between a single exciton and an electron produced by ionization of the dopant donor. These results imply that a better understanding of the effects of surface chemistry on film doping, NC carrier dynamics, and inter-NC interactions is necessary to build solar energy conversion devices that can harvest the multiple carriers produced by MEG. Our results show that the MEG efficiency is very sensitive to the condition of the NC surface and suggest that the wide range of MEG efficiencies reported in the recent literature may be a result of uncontrolled differences in NC surface chemistry.

Li Q., Liu C., Zang L., Gong Q., Yu X., Cao C.

We report the nonlinear absorption, nonlinear scattering, nonlinear refraction, and optical limiting effects in two semiconductor suspensions formed by nanowires of Bi2S3 and nanometer-sized hollow spheres of CuS with nanosecond incident lasers when the incident photon energy (2.33 eV) is above the bandgap. The nonlinear scattering and nonlinear absorption processes were regarded as the mechanisms of the optical limiting effects. With the large positive nonlinear refractive index of those suspensions given by our Z-scan studies, we believed that the nonlinear variation of the refractive index mismatch is one of the reasonable origins of nonlinear scattering. Then, based on the combined effects of nonlinear scattering and absorption, the broadband optical limiting effects in those suspensions were observed. Two Bi2S3 nanowires with different morphologies have similar optical limiting properties.

Pendyala N.B., Koteswara Rao K.S.

We report the type and nature of the surface states in PbS quantum dots grown in poly vinyl alcohol by the colloidal technique. Mercaptoethanol $(C_2H_5OSH)$ capping and the molar ratio of Pb:S were used as parameters to understand the origin of the surface state related photoluminescence. From absorption and photoluminescence measurements, it was observed that increasing Lead concentration resulted in bigger nanoparticles with broad size distribution. However, the increase in sulfur concentration helped in the formation of smaller nanoparticles with narrow size distribution. Passivation studies also revealed that the origin of the bands below 1.1 eV was sulfur related. Thus these experiments indicated that sulfur played an important role, not only in size selectivity, but also in controlling defects in PbS quantum structures. Temperature dependent PL studies on different samples with various Pb:S molar ratios and with mercaptoethanol treated gave an insight into the nature of the surface states. Based on these results, we explain the origin of the surface states and proposed a model for different PL bands. The observed temperature-dependent trends of PL intensity (decreasing in Pb:S::1:1, increasing in S terminated and anomalous behavior in samples with excess of Pb) is attributed to the dominant midgap states and the results are consistent with carrier redistribution and recombination statistics involved in the quantum structures.

Du M., Erwin S.C., Efros A.L.

We propose a framework for describing the impurity doping of semiconductor colloidal nanocrystals. The model is applicable when diffusion of impurities through the nanocrystal is sufficiently small that it can be neglected. In this regime, the incorporation of impurities requires that they stably adsorb on the nanocrystal surface before being overgrown. This adsorption may be preempted by surfactants in the growth solution. We analyze numerically this competition for the case of Mn doping of CdSe nanocrystals. Our model is consistent with recent experiments and offers a route to the rational optimization of doped colloidal nanocrystals.

Asunskis D.J., Bolotin I.L., Hanley L.

Lead sulfide nanocrystals were grown in toluene solutions of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene, polyethylene, poly(1-butene), poly(1-decene), and polystyrene. The PbS nanocrystal diameter was controlled by the polymer used for synthesis and ranged from 4 to 9 nm. Strong nonlinear absorption was observed in open aperture Z-scan measurements of solutions containing the synthesized nanocomposites. The best fit to the experimental data gave high values for βeff in the range of 100−200 cm/GW. The nonlinear absorption of these polymer grown nanocrystals was not observed in nanocrystals grown in polyvinyl alcohol or with oleic acid capping. These differences in nonlinear optical absorption were due to surface chemistry, as the polymer synthesized nanocrystals were free of strongly bonding groups such as surfactant capping or hydroxyl groups. The βeff values in these materials also showed no dependence on nanocrystal size.

Li Q.S., Liu C.L., Zang L.Y., Gong Q.H., Yu X.L., Cao C.B.

Nanoparticles of lead sulfide (PbS) have been prepared by a surfactant micelle template-inducing reaction. These nanoparticles were dispersed in ethanol and investigated for their nonlinear optical properties by nanosecond laser pulses. Broadband optical limiting effects have been observed at the wavelengths of 355, 1064, and 430 nm and every 30 nm from 470 to 620 nm. The nonlinear light-scattering process in the suspensions of PbS nanoparticles was characterized at the wavelength of 532 nm. Nonlinear absorption and nonlinear scattering were deduced to be the mechanisms for the broadband optical limiting performance. The origins of the nonlinear scattering were discussed.

Istrate E., Hoogland S., Sukhovatkin V., Levina L., Myrskog S., Smith P.W., Sargent E.H.

We report transient absorption saturation measurements on lead sulfide colloidal nanocrystals at the first and second exciton energies and fit the results to a model incorporating intraband and interband relaxation processes. We study in detail the Auger recombination from the first excited state, which takes place when more than one electron−hole pair is excited in a dot. We find an Auger coefficient of 4.5 × 10-30 cm6/s for dots of 5.5 nm diameter, and observe saturation of the absorption bleaching when the (8-fold degenerate) first level is filled. We develop a model for the absorption dynamics using Poisson statistics and find a good fit with our experimental measurements.

King S.M., Chaure S., Doyle J., Colli A., Ferrari A.C., Blau W.J.

We present evidence for scattering induced optical limiting in a suspension of Si/SiO2 nanostructures in an isopropanol solution. The structures were studied by open and closed aperture z-scan with 6 ns Gaussian pulses at 532 nm from a frequency doubled, Q switched Nd:YAG laser. The linear absorption of the nanostructures displayed evidence of quantum confinement. An intensity dependent scattering study illustrated the contribution to optical limiting from scattering mechanisms. We propose that optical limiting is occurring at scattering centres, most likely plasmas, which are formed through ionisation of the SiO2 outer shells.

Li C., Shi G., Xu H., Guang S., Yin R., Song Y.

PbS nanorods were synthesized by surfactant-assisted homogenous hydrolysis. The products were characterized by UV–vis spectrophotometer, X-ray powder diffraction (XRD) and transmission electron microscope (TEM). PbS nanorods were measured by the Z-scan technique to investigate the third-order nonlinear optical (NLO) properties. The result of the NLO measurements shows that the PbS nanorods have the third-order nonlinear optical properties of both NLO absorption and NLO refraction with self-focusing effects. The nonlinear absorption coefficient and refractive index of the PbS nanorods are 2.16 × 10− 9 m/W and 3.52 × 10− 16 m2/W respectively.

Kurian P.A., Vijayan C., Sandeep C.S., Philip R., Sathiyamoorthy K.

Strong nonlinear absorption is observed in nanocomposite films containing PbS nanocrystals of mean size of 3.3 nm stabilized in a commercial poly(vinyl acetate) glue by a novel and simple chemical route of synthesis. A significant blueshift of the optical absorption edge indicates strong quantum confinement. The mean nanocrystal size was characterized by x-ray diffraction and transmission electron microscopy. The surface structure of nanocrystals is analysed using infrared spectroscopy. The excitonic transitions are probed by photoacoustic spectroscopy and the results are analysed on the basis of theoretical calculations using envelope function formalism. Results of open aperture z-scan experiments suggest a model involving saturable absorption followed by two-photon absorption at a lower concentration while the data for a higher concentration fitted saturable absorption followed by three-photon absorption. Free carrier absorption due to two-photon-assisted excited state absorption appears to be the predominant mechanism of optical nonlinearity.

Zhang Y., Zhao J., Lv M., Dai E., Zhao Q., Wang M., Wang X.

Mamiyev Z., Balayeva N.O.

The primary motivation behind efforts to understand the properties of lead sulfide (PbS) is its technological importance as the foundation of modern semiconductor optoelectronics. PbS nanocrystals (NCs) show remarkable versatility and promise for application in both conventional optical devices and the new generation of nano-electronics and nano-optoelectronics because of their particular structure and size-related properties. Although PbS NCs have been the subject of intense fundamental and applied studies, no comprehensive evaluation of the published data has been documented so far. Undoubtedly, providing an overview of the published research will help further development in PbS-related research and will certainly make it a step forward in the application. Therefore, this review paper discusses the preparation, structural, electronic and optical properties of PbS NCs in light of recent progress in nanotechnology. We first survey the fundamental properties of PbS NCs and then mainly focus on size-dependent properties. In this regard, the recent breakthrough in tuning the properties of PbS NCs employing doping, ligands, capping and hybridizing with different materials, and interaction with embedded media has been particularly discussed. The review concludes with the enhanced performance of the PbS NCs towards future optoelectronic devices and biological and chemical applications.

Zvyagin A.I., Chevychelova T.A., Chirkov K.S., Smirnov M.S., Ovchinnikov O.V.

The size dependence of the nonlinear optical response of hydrophilic colloidal PbS quantum dots, passivated with thioglycolic acid (PbS/TGA QDs) with an average size of 3.0, 3.5, 4.5 nm in the field of 10 ns laser pulses at 532 nm has been analyzed. It is shown that the key mechanism of nonlinear absorption is the reverse saturable absorption, which occurs with involvement of QDs defect trap states. The formation of SiO 2 monolayer on the PbS/TGA QDs surface, using MPTMS makes it possible to significantly increase in the luminescence intensity from 4 to 4.5 times. In this case, the nonlinear absorption coefficient and nonlinear refraction index do not change. The facts indicate the implementation of a nonlinear optical response in the nanosecond probe pulse range on trap states that are not luminescence centers.

Zvyagin A.I., Chevychelova T.A., Chirkov K.S., Smirnov M.S., Ovchinnikov O.V.

A Z-scan study is performed of the nonlinear optical response of hydrophilic colloidal PbS and Ag2S quantum dots with average sizes of 2.2 and 2.9 nm, respectively, passivated with 2-mercaptopropionic acid (2-MPA). A Nd3+:YAG laser is used at wavelengths of 355 and 532 nm and a pulse length of 10 ns. It is found that the PbS/2-MPA and Ag2S/2-MPA colloidal solutions exhibit nonlinear refraction (defocusing) at both wavelengths. The saturation character of the dependence of the coefficient of nonlinear absorption on the intensity of laser radiation is revealed. It is established that localized states (e.g., the center of recombination luminescence) are responsible for both nonlinear optical effects.

Zvyagin A.I., Chevychelova T.A., Grevtseva I.G., Smirnov M.S., Selyukov A.S., Ovchinnikov O.V., Ganeev R.A.

We analyze the features of nonlinear refraction in colloidal solutions containing Ag2S quantum dots capped with thioglycolic acid (Ag2S/TGA) or L-cysteine (Ag2S/L-Cys), with the quantum-dot (QD) sizes ranging from 1.7 to 5.5 nm and their concentration in the solution being 0.1%. The Z-scans of the samples were obtained using 10 ns pulses of 532 nm radiation. For samples with various average sizes and positions of the characteristic feature in the optical density spectrum, both self-focusing and self-defocusing of radiation were observed. The contribution of thermal refraction to the recorded Z-scans was estimated, and the impact of thermal effects on the observed nonlinear refraction was shown to be negligible. The solutions of the Ag2S/TGA QDs with the ground state exciton absorption peaked at 590 nm exhibit self-focusing. The samples with recombination luminescence and the ground state exciton absorption peaked at 750 nm reveal self-defocusing. We show that nonlinear refraction in the case of nanosecond pulses is predominantly nonthermal in nature and is due to absorption by free charge carriers, as well as due to the band filling effect in the Ag2S QDs, which involves localized states of impurities.

Zvyagin A.I., Smirnov M.S., Ovchinnikov O.V.

The nonlinear optical response of thiazine dyes during association with colloidal CdS quantum dots (QDs) applying the z-scan technique at wavelength of 532 nm with use 10 ns pulses of Nd:YAG laser second harmonic were studied. Increasing in nonlinear absorption intensity in hybrid associates in comparison with the initial components were found. Moreover, UV–vis absorption spectrum shows the effective formation of dyes H-aggregates during association with colloidal QDs. Two-photon mechanism of observed nonlinear absorption in hybrid associates is proposed, based on a sharper dependence of the normalized transmittance on the incident intensity (distance between the sample and focal plane of focusing lens). And for pure solutions, the reverse saturated absorption is preferred nonlinear absorption mechanism. The nonlinear absorption coefficients for MB and AzA in the hybrid associates were found. They are β ≈ 8.4 × 10 −9 cm W -1 и ≈ 1.34 × 10 -8 cm W -1 , respectively. It was shown that it is possible to control the parameters of nonlinear absorption of thiazine dyes via the hybrid association.

Li Q., Zhang Y., Chen Z., Pan X., Zhang Z., Zhu J., Zhu X.

Novel synthesis of selenium containing polymers with pre-determined structures and applications thereof.

Skurlov I.D., Ponomareva E.A., Ismagilov A.O., Putilin S.E., Vovk I.A., Sokolova A.V., Tcypkin A.N., Litvin A.P.

Due to their high resonant third-order nonlinear response, lead sulfide quantum dots (QDs) are potential materials for applications in the field of nonlinear optics. In this paper, we implement the Z-scan method to study the resonant nonlinear response of lead sulfide QDs in colloidal solutions. We managed to measure the purely intrinsic resonant nonlinear response, free of thermal contribution. We report that the lead sulfide QD third-order nonlinear response per QD shows an unconventional increase. A figure of merit for QD nonlinearity grows as a power function with a factor of 2.9.

Kondratenko T.S., Smirnov M.S., Ovchinnikov O.V., Zvyagin A.I., Ganeev R.A., Grevtseva I.G.

Decreasing in the self-defocusing threshold at wavelength of 532 nm using 10 ns pulses of Nd:YAG laser second harmonic radiation for hybrid associates of Ag2S (2.0 nm) quantum dots (QDs) with erythrosine molecules (Ery) was shown with use the Z-scan method. The obtained result is attributed to the manifestation of hybrid properties due to the interaction of QDs and Ery molecules in associate. It can be explained using the data on the hybrid associates spectral-luminescent properties. The hybrid association with Ery molecules blocks Ag2S QDs IR luminescence under excitation with radiation from the region of dye absorption. Based on the absence of noticeable changes in the luminescence lifetime in the nanosecond range, as well as position of HOMO–LUMO dye states with respect to the size quantization states of Ag2S QDs, it was concluded that electron is rapidly transferred from Ery to the size quantization states in the conduction band. It leads to change the degree of QDs band filling.

Ganeev R.A.

Any new synthesized semiconductor nanoparticles or quantum dots (QDs) require to be examined under different conditions using laser pulses of variable energies, wavelengths and durations to understand the nonlinear optical mechanisms and distinguish their attractive properties for practical applications. Among various QDs, the metal sulfides took special attention due to their advanced nonlinear optical properties. Additionally, laser ablation can readily produce plasmas containing QDs that could be used for different applications. One of them is that they can be used as emitters for harmonic generation from ultrashort laser pulses. Here we review recent studies of different low- and high-order optical nonlinearities of metal sulfide QDs, such as optical limiting, two-photon, saturable and reverse saturable types of nonlinear absorption, nonlinear refraction, and generation of high-order harmonics in extreme ultraviolet range.

Kondratenko T.S., Zvyagin A.I., Smirnov M.S., Grevtseva I.G., Perepelitsa A.S., Ovchinnikov O.V.

Abstract The relationship of the spectral luminescent and nonlinear optical properties of Ag2S quantum dots (QDs), passivated with thioglycolic acid (Ag2S QDs/TGA) and Ag2S QDs, stabilized by gelatin (Ag2S QDs/Gel) with average sizes from 1.5 nm to 2.1 nm is found. Colloidal Ag2S QDs/TGA with different sizes, having the exciton (620 nm) and recombination (660 nm, 880 nm and 1205 nm) luminescence were studied using the methods of luminescent spectroscopy, luminescence decay, and Z-scans technique at the wavelengths of 532 nm using 10 ns pulses. Resonant two-photon transitions of reverse saturated absorption (RSA) were found for samples with recombination luminescence. In addition, the nonlinear refraction of thermal nature was observed using Z-scan method. Predominantly weak two-photon absorption (TPA) and nonlinear refraction (NR) of non-thermal nature occur for Ag2S/TGA quantum dots with exciton luminescence. The luminescence lifetimes support this conclusion. The differences in luminescence lifetimes of samples are associated with different state of interfaces for colloidal Ag2S QDs/TGA with recombination and exciton luminescence. This conclusion was confirmed by FTIR spectroscopy of colloidal Ag2S QDs/TGA solutions, synthesized under different conditions. RSA is characteristic of samples with a noticeable concentration of trap states acting as recombination luminescence centers (660 nm). The maximum effect was noted for Ag2S QDs with recombination luminescence in the region of 660 nm, 880 nm, 1205 nm. The fast component of this luminescence decay has a time constant of about 0.8 ns. It decay 5 times for the first 10 ns. In the case of Ag2S QDs/TGA with exciton luminescence (620 nm) there are no appropriate conditions for RSA. The fast component of luminescence decay has a time constant of 0.8 ns. The luminescence intensity decreases 50 times for the first 10 ns. In this case, TPA and nonlinear refraction of non-thermal nature take place, and RSA is absent.

Zvyagin A.I., Smirnov M.S., Ovchinnikov O.V., Selyukov A.S.

We present the results of the study of the nonlinear optical response observed in hybrid associates of Azure A dye with Zn0.5Cd0.5S colloidal quantum dots upon exposure to 10-ns pulses of the second harmonic (532 nm) of a Nd3+:YAG laser. It is shown that association enhances the absorption saturation in the dye by a factor of 4.5, which is caused by charge phototransfer between the components of the hybrid associate.

Fu Y., Ganeev R.A., Zhao C., Rao K.S., Maurya S.K., Yu W., Zhang K., Guo C.

We demonstrate strong optical nonlinearities of silver sulfide (Ag2S) quantum dots (QDs) in the ultraviolet range. The 4-nm Ag2S QDs were prepared by chemical method and analyzed using picosecond (800 nm and 400 nm, 200 ps) and femtosecond (800 nm and 400 nm, 60 fs) laser pulses. Our Z-scan measurements show that these QDs have large nonlinear absorption coefficient (~ 10−3 cm W−1) at 400 nm. We also demonstrate the transient absorption and optical limiting in Ag2S QDs. Variations of the signs of nonlinear refractive indices and nonlinear absorption coefficients of QDs are demonstrated by changing pulse width and wavelength of probe radiation.

Kondratenko T.S., Grevtseva I.G., Zvyagin A.I., Ovchinnikov O.V., Smirnov M.S.

Spectral, kinetic, and nonlinear optical regularities that demonstrate the exchange of electronic excitations between the components of hybrid associates of Ag2S colloid quantum dots (1.7–1.8 nm) in gelatin with molecules of thiazine dyes (Ds) are found. When the IR luminescence of Ag2S quantum dots (QDs) is excited by radiation from the thionine absorption region, its enhancement due to nonradiative resonant energy transfer is observed. The association with methylene-blue molecules blocked the IR luminescence of Ag2S QDs upon its excitation by radiation from the absorption region of the dye due to the transfer of charge carriers. It is demonstrated that the hybrid association of thionine molecules and Ag2S QDs adversely affects the nonlinear optical properties of the latter, which manifests itself in inverse saturated absorption by the action of 10-ns second-harmonic pulses (532 nm) of a Nd3+:YAG laser. For the associates of Ag2S QDs with methylene-blue molecules, the radiation focusing caused by the transfer of charge carriers from the dye and the change in the population of small traps in nanocrystals is found. It is concluded that the direction of the transfer of electronic excitations and the photophysical processes in these objects are determined by the mutual arrangement of the HOMO–LUMO levels of the dye with respect to the levels of dimensional quantization of the Ag2S QDs.

Xing C., Xie Z., Liang Z., Liang W., Fan T., Ponraj J.S., Dhanabalan S.C., Fan D., Zhang H.

Investigations into 2D nanomaterials are of considerable significance from both an academic and industrial point of view. The present work addresses, for the first time, the fabrication of 2D nonlayered ultrathin selenium through a facile liquid-phase exfoliation method. The results reveal that the as-prepared 2D Se nanosheets are 40–120 nm in lateral dimension and 3–6 nm in thickness. The nanosheets exhibit a trigonal crystalline phase similar to their bulk counterpart, indicating the conservation of the crystalline features during the exfoliation procedure. The successful preparation of 2D Se nanosheets from nonlayered bulk Se can be ascribed to two kinds of anisotropy: (1) that of crystalline bulk Se with chain-like structures, where strong intrachain SeSe covalent bonds coexist with weak interchain van der Waals forces, and (2) that of probe sonication in the vertical direction. The results also show that the 2D Se nanosheets possess a size-dependent band gap (E g), strong photoluminescence effect and robust, chemical stability under ambient conditions. Furthermore, a 2D Se-nanosheet-based optical modulation device is demonstrated that allows for excellent ultrashort pulse generation of an optical communication band. It is therefore anticipated that 2D Se nanosheets may find significant applications in both photoluminescence and ultrafast photonics.