Gold(I) N-heterocyclic carbene precursors for focused electron beam-induced deposition

• Cristiano Glessi,
• Aya Mahgoub,
• Cornelis W. Hagen and
• Mats Tilset

Beilstein J. Nanotechnol. 2021, 12, 257–269, doi:10.3762/bjnano.12.21

• yielded only traces of gold (2–3 atom % [19]). Within the series of gold acetonate complexes, the highest gold content has been achieved with Au(tfac)Me2 when water was co-injected as an oxidizing agent during the deposition (91 atom % gold) [26]. To circumvent carbon contamination, a series of inorganic

• heavily contaminated with carbon. Therefore, it makes sense to use as little carbon as possible in the design of the molecule [17]. Furthermore, it has been observed that large ligands, such as the methylcyclopentadienyl group in MeCpPtMe3 and acetylacetonate in Au(acac)Me2, do not decompose favourably

• species are classified as NHCs, they all share a common moiety, a carbene carbon stabilized by two α-nitrogen atoms. NHCs are neutral two-electron donors, analogous to the more extensively studied CO and PR3 derivatives. The coordinative capability of the NHCs depends in fact primarily on the sp2

Scanning transmission helium ion microscopy on carbon nanomembranes

• Daniel Emmrich,
• Annalena Wolff,
• Nikolaus Meyerbröker,
• Jörg K. N. Lindner,
• André Beyer and
• Armin Gölzhäuser

Beilstein J. Nanotechnol. 2021, 12, 222–231, doi:10.3762/bjnano.12.18

• principle is based on a secondary electron conversion holder with an exchangeable aperture strip allowing its acceptance angle to be tuned from 3 to 98 mrad. The contrast mechanism and performance were investigated using freestanding nanometer-thin carbon membranes. The results demonstrate that the detector

• energy-filtered transmission electron microscopy measurements. Keywords: carbon nanomembranes; dark field; helium ion microscopy (HIM); scanning transmission ion microscopy (STIM); SRIM simulations; Introduction Throughout the past decade, the helium ion microscope (HIM) has emerged as a versatile

• -field scanning transmission ion microscopy (STIM) holder. The holder design is based on the concept of a SE conversion holder. The holder can easily be implemented into any existing HIM. It is mounted on the sample stage without modifications to the microscope. Carbon nanomembranes (CNMs) serve as test

A review on the biological effects of nanomaterials on silkworm (Bombyx mori)

• Sandra Senyo Fometu,
• Guohua Wu,
• Lin Ma and
• Joan Shine Davids

Beilstein J. Nanotechnol. 2021, 12, 190–202, doi:10.3762/bjnano.12.15

• observed [32]. TiO2 NPs have also been documented to increase the germination of lettuce seeds [33]. Recent reports indicate that multi-walled carbon nanotubes (MWCNTs) also increased the germination of tomato seeds by increasing the seed water uptake [34]. These reports of nanomaterials improving the

• al. [41] reported that the dietary uptake of carbon-based nanomaterials (fullerene C60, carbon black, single-walled nanotubes, multi-walled nanotubes) had no toxic effect on the growth and development of Drosophila melanogaster (fruit fly) from the egg stage to the adult form. However, the dietary

• uptake of carbon black and single-walled nanotubes caused impairment in the locomotor function of these flies. Demir et al. [42] reported that Ag NPs (0.1, 1, 5, and 10 mM) induced genotoxicity in the wing spot assay of fruit flies via somatic recombination. Carmona et al. [43] reported cytotoxic effects

Imaging of SARS-CoV-2 infected Vero E6 cells by helium ion microscopy

• Natalie Frese,
• Patrick Schmerer,
• Martin Wortmann,
• Matthias Schürmann,
• Matthias König,
• Michael Westphal,
• Friedemann Weber,
• Holger Sudhoff and
• Armin Gölzhäuser

Beilstein J. Nanotechnol. 2021, 12, 172–179, doi:10.3762/bjnano.12.13

• ethanol (50%, 70%, 95%, 99.5% (2×)), transferred to water-free acetone and critical point dried in carbon dioxide. HIM was performed with an Orion Plus microscope (Carl Zeiss) at an acceleration voltage of about 36 kV and a working distance of 20 mm. The spot control was set to 6 to obtain a beam current

• allowed imaging of Figure 2c3 without any external charge compensation. From the quality of this image, it can be inferred that the deposited carbon layer rendered the sample sufficiently conductive. However, small structures are still visible on the membrane surface, which may originate from surface

• topography or material contrast. The deposited carbon film is presumably thinner than typical conductive metal or carbon coatings for SEM imaging, and it does not show any surface masking and clustering as seen on the gold substrate in the upper left of Figure 2b2. The energy of the incident hydrocarbons is

Paper-based triboelectric nanogenerators and their applications: a review

• Jing Han,
• Nuo Xu,
• Yuchen Liang,
• Mei Ding,
• Junyi Zhai,
• Qijun Sun and
• Zhong Lin Wang

Beilstein J. Nanotechnol. 2021, 12, 151–171, doi:10.3762/bjnano.12.12

• ., metal nanowires, conducting polymers, carbon nanotube (CNT) inks, multiwall carbon nanotube (MWCNT) inks, and reduced graphene oxide) [69][70][71][72][73][74][75][76][77][78][79][80][81][82], can be easily absorbed or used as a coating layer on the surface of the paper due to its wettability and

• changing the concentration of the conductive precursors (e.g., AgNWs, carbon nanotubes, and reduced graphene oxide). A special nanoscale paper composed of nanocellulose, which is easy to be synthesized/chemically modified/doped, has attracted great attention in recent years. Generally, cellulose-based

• et al. reported a cut-paper-based self-charging power unit (PC-SCPU) by combining TENGs and the SC mode, which is capable of simultaneously harvest and store energy from body movements [39]. Zhou et al. also proposed an all-in-one stretchable self-charging power unit based on folded carbon paper

A review on the green and sustainable synthesis of silver nanoparticles and one-dimensional silver nanostructures

• Sina Kaabipour and
• Shohreh Hemmati

Beilstein J. Nanotechnol. 2021, 12, 102–136, doi:10.3762/bjnano.12.9

Fusion of purple membranes triggered by immobilization on carbon nanomembranes

• René Riedel,
• Natalie Frese,
• Fang Yang,
• Martin Wortmann,
• Raphael Dalpke,
• Daniel Rhinow,
• Norbert Hampp and
• Armin Gölzhäuser

Beilstein J. Nanotechnol. 2021, 12, 93–101, doi:10.3762/bjnano.12.8

• Max Planck Institute of Biophysics, Department of Structural Biology, Max-von-Laue-Str. 3, D-60438 Frankfurt, Germany 10.3762/bjnano.12.8 Abstract A freestanding ultrathin hybrid membrane was synthesized comprising two functional layers, that is, first, a carbon nanomembrane (CNM) produced by

• PM on a solid support. The resulting hybrid membrane has a potential application as a light-driven two-dimensional proton-pumping membrane, for instance, for light-driven seawater desalination as envisioned soon after the discovery of PM. Keywords: bacteriorhodopsin; carbon nanomembrane

• study protein behavior and also do not provide the means to construct an oriented PM monolayer for potential technical applications [33][34]. In this work, we report the successful assembly of a unidirectionally oriented PM quasi-monolayer on an ultrathin carbon nanomembrane (CNM) substrate. CNMs are

The role of gold atom concentration in the formation of Cu–Au nanoparticles from the gas phase

• Yuri Ya. Gafner,
• Svetlana L. Gafner,
• Darya A. Ryzkova and
• Andrey V. Nomoev

Beilstein J. Nanotechnol. 2021, 12, 72–81, doi:10.3762/bjnano.12.6

• application in the field of catalysis and optics. Thus, it has been shown that, in heterogeneous catalysis, Cu–Au nanoalloys usually exhibit a synergistic effect in oxidative reactions and the preferential oxidation of carbon monoxide (CO) to carbon dioxide at ambient temperature as well as in the selective

• amorphous carbon or magnesium oxide substrates by the laser evaporation of a bulk alloy with various stoichiometric compositions (Cu–Au, Cu3Au, and Au3Cu). An analysis of individual clusters carried out by using electron diffraction and high-resolution transmission electron microscopy (HRTEM) showed that Cu

• –Au clusters are formed with chemical compositions corresponding to the composition of the evaporated material [14]. In the case of cluster deposition onto amorphous carbon, various cluster morphologies were observed, such as cuboctahedral and decahedral. For clusters supported on a MgO substrate

ZnO and MXenes as electrode materials for supercapacitor devices

• Ameen Uddin Ammar,
• Ipek Deniz Yildirim,
• Feray Bakan and
• Emre Erdem

Beilstein J. Nanotechnol. 2021, 12, 49–57, doi:10.3762/bjnano.12.4

• changes in the Raman spectrum. In particular, this method is used for carbon materials to detect the so-called D-band, which belongs to sp3-hybridized carbon. For the occurrence of the D-band either C dangling bonds on the surface, C interstitials, or C vacancies need to exist. Raman spectra of metal

• oxides are more complicated than those of carbon materials because there are many peaks for each mode. The most prominent peak in Raman spectra of ZnO is E2(high) for ZnO crystals. Once the size of the ZnO crystallites is below 50 nm, broadening, softening, and a significant shift is observed for this

• supercapacitors [15]. Until now, carbon (high surface area), metal oxides (high specific capacitance and low resistance), and conducting polymers are used as electrode materials. The issues of cost, stability, and lifetime are not resolved yet [15]. Graphene seems like one of the possible main electrode materials

Effect of different silica coatings on the toxicity of upconversion nanoparticles on RAW 264.7 macrophage cells

• Cynthia Kembuan,
• Helena Oliveira and
• Christina Graf

Beilstein J. Nanotechnol. 2021, 12, 35–48, doi:10.3762/bjnano.12.3

• UCNPs, was dried on a carbon-coated copper grid (Cu 400 mesh, Quantifoil®: 100 carbon support films). The images were analyzed using the software FIJI. At least 300 particles per sample were analyzed. Dynamic light scattering and electrophoretic light scattering The DLS and ELS measurements were

Bio-imaging with the helium-ion microscope: A review

• Matthias Schmidt,
• James M. Byrne and
• Ilari J. Maasilta

Beilstein J. Nanotechnol. 2021, 12, 1–23, doi:10.3762/bjnano.12.1

• preparation stage with a supercritical fluid, such as carbon dioxide (CO2). The temperature and pressure around the sample are raised to the critical point, 31 °C and 7.4 MPa (1073 psi), at which point CO2 becomes supercritical. The pressure is then lowered at constant temperature returning the CO2 to a

• materials, such as carbon, platinum, or gold. Coating the samples can, however, result in minute changes to surface features which are only visible at very high resolution. One of the main advantages of HIM is the ability to neutralise charge by implementation of an electron flood gun. This flood gun

Free and partially encapsulated manganese ferrite nanoparticles in multiwall carbon nanotubes

• Saja Al-Khabouri,
• Salim Al-Harthi,
• Toru Maekawa,
• Mohamed E. Elzain,
• Ashraf Al-Hinai,
• Ahmed D. Al-Rawas,
• Abbsher M. Gismelseed,
• Ali A. Yousif and
• Myo Tay Zar Myint

Beilstein J. Nanotechnol. 2020, 11, 1891–1904, doi:10.3762/bjnano.11.170

• manganese ferrite nanoparticles inside the tubes is observed as a shift in the X-ray diffraction peaks and as an increase in stress, hyperfine field, and coercivity when compared to free manganese ferrite nanoparticles. On the other hand, a strong charge transfer from the multiwall carbon nanotubes is

• ferrite nanoparticles in comparison to free manganese ferrite nanoparticles, which leads to an enhancement of the metallic properties. Keywords: carbon nanotubes; charge transfer; manganese ferrite; metallic nanoparticles; partial encapsulation; stress; surface; Introduction Since the discovery of

• carbon nanotubes (CNTs), researchers have been interested in functionalizing, inserting, and encapsulating materials inside their inner cavities. Processes related to functionalizing and inserting nanomaterials (i.e., directly implanting as-synthesized nanomaterials into CNTs) have been widely reported

Kondo effects in small-bandgap carbon nanotube quantum dots

• Patryk Florków,
• Damian Krychowski and
• Stanisław Lipiński

Beilstein J. Nanotechnol. 2020, 11, 1873–1890, doi:10.3762/bjnano.11.169

• Patryk Florkow Damian Krychowski Stanislaw Lipinski Department of Theory of Nanostructures, Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17,60-179 Poznań, Poland 10.3762/bjnano.11.169 Abstract We study the magnetoconductance of small-bandgap carbon nanotube

• close to perpendicular. When the field approaches a transverse orientation a crossover from SU(2) or SU(3) symmetry into SU(4) is observed. Keywords: carbon nanotubes; Kondo effect; mesoscopic transport; quantum dots; valleytronics; Introduction Due to their remarkable electronic, transport

• , mechanical, and magnetic properties carbon nanotubes (CNTs) are of great interest in molecular electronics and spintronics with potential applications, for example, as field-effect transistors, nanoelectromechanical devices, logic gates, spin valves, spin diodes, and spin batteries [1][2][3][4][5][6][7][8][9

Towards 3D self-assembled rolled multiwall carbon nanotube structures by spontaneous peel off

• Jonathan Quinson

Beilstein J. Nanotechnol. 2020, 11, 1865–1872, doi:10.3762/bjnano.11.168

• individual nanomaterials can be a challenging task. However, it opens up opportunities for the production of increasingly complex nanostructures. Unusual rolled multiwall carbon nanotube structures are synthesized here by simply inducing a change of precursor composition during the growth of multiwall carbon

• nanotube forests. The multiwall carbon nanotube structures are comprised of nitrogen-doped and undoped sections, and are obtained via a detailed peel off and roll mechanism. These results open new doors for the development of increasingly complex nanostructures. Keywords: chemical vapor deposition

• ; multiwall carbon nanotubes; nitrogen doping; peel off; rolled carbon nanotubes; Introduction Carbon nanotubes (CNTs) are popular materials used in various applications [1]. These tubular hollow carbon nanomaterials have proven to be useful in multiple scientific fields [2][3][4][5][6]. Complex structures

Scanning transmission imaging in the helium ion microscope using a microchannel plate with a delay line detector

• Eduardo Serralta,
• Nico Klingner,
• Olivier De Castro,
• Michael Mousley,
• Santhana Eswara,
• Serge Duarte Pinto,
• Tom Wirtz and
• Gregor Hlawacek

Beilstein J. Nanotechnol. 2020, 11, 1854–1864, doi:10.3762/bjnano.11.167

• available in an open access repository [38]. Results and Discussion Mass-thickness contrast Bright-field and dark-field contrast In Figure 2, we show images of a carbon film under lacey carbon using the SE imaging mode (Figure 2a), BF STIM (Figure 2b), and ADF STIM (Figure 2c). The data for Figure 2b and

• Figure 2c was acquired in a single acquisition after Figure 2a, and the two images were generated selecting from the data two appropriate angular sectors of the detector. The areas of the sample where the carbon film has the lacey carbon on top show different gray levels compared to the areas where there

• partially filled with carbon and yield some SE signal. The bigger pores appear dark in dark-field mode (Figure 4c–e) because there is no scattering. Counterintuitively, the same pores appear dark in bright-field mode (Figure 4b) as well. This behavior can be explained by the intensity of the full primary

Self-standing heterostructured NiC_x-NiFe-NC/biochar as a highly efficient cathode for lithium–oxygen batteries

• Shengyu Jing,
• Xu Gong,
• Shan Ji,
• Linhui Jia,
• Bruno G. Pollet,
• Sheng Yan and
• Huagen Liang

Beilstein J. Nanotechnol. 2020, 11, 1809–1821, doi:10.3762/bjnano.11.163

• , 314001, China Low Carbon Energy Institute, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China Hydrogen Energy and Sonochemistry Research Group, Department of Energy and Process Engineering, Faculty of Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim

• catalysts is highly desirable for practical applications in lithium–oxygen batteries. Herein, a heterostructure of NiFe and NiCx inside of N-doped carbon (NiCx-NiFe-NC) derived from bimetallic Prussian blue supported on biochar was developed as a novel self-standing cathode for lithium–oxygen batteries. The

• specific discharge capacity of the best sample was 27.14 mAh·cm−2 at a stable discharge voltage of 2.75 V. The hybridization between the d-orbital of Ni and s and p-orbitals of carbon in NiCx, formed at 900 °C, enhanced the electrocatalytic performance due to the synergistic effect between these components

Nanomechanics of few-layer materials: do individual layers slide upon folding?

• Ronaldo J. C. Batista,
• Rafael F. Dias,
• Ana P. M. Barboza,
• Alan B. de Oliveira,
• Taise M. Manhabosco,
• Thiago R. Gomes-Silva,
• Matheus J. S. Matos,
• Andreij C. Gadelha,
• Cassiano Rabelo,
• Luiz G. L. Cançado,
• Ado Jorio,
• Hélio Chacham and
• Bernardo R. A. Neves

Beilstein J. Nanotechnol. 2020, 11, 1801–1808, doi:10.3762/bjnano.11.162

• carbon nanotubes [13][14]. The interlayer slip is also intimately related to the dependence of κ on the thickness or the number of layers of a 2D material, which, in the case of very thin 2D materials, may be very different from that obtained from classical theories [12]. The quantification and

• )/2 and d0 = d − h, are parameters for the proposed continuum model. Carbon atom positions (gray circles) in cross sections of folded edges in (a) monolayer graphene and (b) three-layered graphene, as obtained through MD simulations. In both panels the red and black lines that superimpose the atomic

• Pessoal de Nível Superior (CAPES); Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG); the Brazilian Institute of Science and Technology (INCT) in Carbon Nanomaterials; and Universidade Federal de Ouro Preto (UFOP-Grant Custeio).

Electron beam-induced deposition of platinum from Pt(CO)₂Cl₂ and Pt(CO)₂Br₂

• Aya Mahgoub,
• Hang Lu,
• Rachel M. Thorman,
• Konstantin Preradovic,
• Titel Jurca,
• Lisa McElwee-White,
• Howard Fairbrother and
• Cornelis W. Hagen

Beilstein J. Nanotechnol. 2020, 11, 1789–1800, doi:10.3762/bjnano.11.161

• deposits contained halogen species and little or no carbon, while the SEM deposits contained only small amounts of halogen species but high carbon content. Results from this study highlight the effect that deposition conditions can have on the composition of deposits created by FEBID. Keywords: energy

• main challenges associated with FEBID is the typical low purity of the deposits. Many FEBID precursors are organometallic, leading to high carbon content in the deposit [6][8][9]. Often, unwanted fragments of the precursor molecules remain in the deposits [10]. Some precursors perform better in this

• current during EDX was 5 nA and the sample was mounted at a working distance of 7.5 mm and tilted by 35° to maximise the EDX signal. The system was plasma cleaned before the EDX measurements were taken to minimize carbon contamination. UHV deposition The UHV deposition experiments were performed by

PEG/PEI-functionalized single-walled carbon nanotubes as delivery carriers for doxorubicin: synthesis, characterization, and in vitro evaluation

• Shuoye Yang,
• Zhenwei Wang,
• Yahong Ping,
• Yuying Miao,
• Yongmei Xiao,
• Lingbo Qu,
• Lu Zhang,
• Yuansen Hu and
• Jinshui Wang

Beilstein J. Nanotechnol. 2020, 11, 1728–1741, doi:10.3762/bjnano.11.155

• of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, P. R. China 10.3762/bjnano.11.155 Abstract Single-walled carbon nanotubes (SWCNTs) have attracted great interest regarding drug-delivery applications. However, their application has been limited by some inherent disadvantages

• drugs. Keywords: antitumor activity; cellular uptake; PEG functionalization; PEI functionalization; poly(ethylene glycol) (PEG); polyethylenimine (PEI); single-walled carbon nanotubes; Introduction To date, chemotherapy is the most common therapy for cancer treatment. However, the inability of

• the distribution in normal tissues, and to improve the biological half-lives [3][4][5][6]. Carbon nanotubes (CNTs) have attracted great interest for biomedical applications, including the delivery of bioactive molecules such as drugs, the targeted cancer therapy, and biological imaging, because of

Out-of-plane surface patterning by subsurface processing of polymer substrates with focused ion beams

• Serguei Chiriaev,
• Luciana Tavares,
• Vadzim Adashkevich,
• Arkadiusz J. Goszczak and
• Horst-Günter Rubahn

Beilstein J. Nanotechnol. 2020, 11, 1693–1703, doi:10.3762/bjnano.11.151

• mineral-organic polymer (its structure includes both carbon and silicon atoms). It is an elastomer and its elasticity can be tuned within a very broad range by changing the degree and the type of polymerization and by post-curing treatments [19][20]. The high and easily tunable elasticity, combined with

• certain fluence, the material capacity to shrink decreases, which explains the saturation effect in the case of irradiation with He+ ions. In the case of PMMA, the high irradiation fluence results in the formation of a compact carbon-rich material that can no longer shrink [13]. Taking this into

The influence of an interfacial hBN layer on the fluorescence of an organic molecule

• Christine Brülke,
• Oliver Bauer and
• Moritz M. Sokolowski

Beilstein J. Nanotechnol. 2020, 11, 1663–1684, doi:10.3762/bjnano.11.149

• between 1,250 and 1,650 cm−1 [41][42][43]. The spectral positions of most of these modes are about constant for PTCDA adsorbed on different substrates [41] or for different film thicknesses [42]. An interesting exception was observed for PTCDA on Ag(111) [42] for the breathing mode of the central carbon

• intramolecular bonds of the core (including the central carbon ring), which causes the higher Raman shift of the RB mode. The flat, saddle-, and arch-like shapes of the molecule on hBN/Cu(111), Ag(111), and KCl/Ag(100) lead to smaller Raman shifts of the RB mode of 12 cm−1, 15 cm−1, and 25 cm−1, respectively

• ” supporting SERS because FLA is shifted in energy, which is less expected for coupling to metallic protrusions. An alternative origin of these defects could be the presence of carbon containing species due to segregation from the Cu bulk. Although this interpretation is speculative, we give some background in

Piezoelectric sensor based on graphene-doped PVDF nanofibers for sign language translation

• Shuai Yang,
• Xiaojing Cui,
• Rui Guo,
• Zhiyi Zhang,
• Shengbo Sang and
• Hulin Zhang

Beilstein J. Nanotechnol. 2020, 11, 1655–1662, doi:10.3762/bjnano.11.148

• Pharmaceutical Group. Graphene powder was purchased from Suzhou Carbon Fung Graphene Technology Company. Ag NWs (60 nm in diameter) were purchased from Lit Nanotech. Ti3C2 MXene was purchased from Beijing Beike New Material Technology Company. Polydimethylsiloxane (PDMS) and curing agent were purchased from

Selective detection of complex gas mixtures using point contacts: concept, method and tools

• Alexander P. Pospelov,
• Victor I. Belan,
• Dmytro O. Harbuz,
• Volodymyr L. Vakula,
• Lyudmila V. Kamarchuk,
• Yuliya V. Volkova and
• Gennadii V. Kamarchuk

Beilstein J. Nanotechnol. 2020, 11, 1631–1643, doi:10.3762/bjnano.11.146

• breath contains oxidizing agents (e.g., nitrogen oxides, carbon oxide, and sulfur oxides), reducing agents (e.g., hydrogen sulfide, ammonia, mercaptans, and organic molecules), and other chemically active components that can participate in various chemical transformations. It is known, for example, that

PTCDA adsorption on CaF₂ thin films

• Philipp Rahe

Beilstein J. Nanotechnol. 2020, 11, 1615–1622, doi:10.3762/bjnano.11.144

• ) surface area. (c) Example of imaging PTCDA on a CaF2/CaF1/Si(111) multilayer in constant-height mode. (f) Model drawing of a single PTCDA molecule (black: carbon, white: hydrogen, red: oxygen atoms). Statistical analysis of (a) the double-lobe orientation on the CaF1 layer and (b) the nearest-neighbour

High-responsivity hybrid α-Ag₂S/Si photodetector prepared by pulsed laser ablation in liquid

• Raid A. Ismail,
• Hanan A. Rawdhan and
• Duha S. Ahmed

Beilstein J. Nanotechnol. 2020, 11, 1596–1607, doi:10.3762/bjnano.11.142

• Ag2S nanoparticles, while the origin of carbon is CTAB. Hall measurement revealed that the Ag2S had a negative Hall coefficient, indicating that it is an n-type semiconductor. This finding agrees well with [39]. Figure 11 shows the dark I–V characteristics of an n-Ag2S/p-Si heterojunction prepared