Direct observation of ferromagnetic spin polarization in gold nanoparticles

Effects of Chiral Polypeptides on Skyrmion Stability and Dynamics

Magnetic skyrmions, topologically stabilized chiral spin textures in magnetic thin films, have garnered considerable interest due to their efficient manipulation and resulting potential as efficient nanoscale information carriers. One intriguing approach to address the challenge of tuning skyrmion properties involves using chiral molecules. Chiral molecules can locally manipulate magnetic properties by inducing magnetization through spin exchange interactions and by creating spin currents. Here, Magneto-Optical Kerr Effect (MOKE) microscopy is used to image the impact of chiral polypeptides on chiral magnetic structures. The chiral polypeptides shift the spin reorientation transition temperature, reduce thermal skyrmion motion, and alter the coercive field locally, enhancing skyrmion stability and thus enabling local control. These findings demonstrate the potential of chiral molecules to address challenges for skyrmion based devices, thus paving the way to applications such as the racetrack memory, reservoir computing and others.

Coexistence of Superconductivity and Magnetic Ordering in the In-Ag Alloy Under Nanoconfinement

The impact of the interface phenomena on the properties of nanostructured materials is the focus of modern physics. We studied the magnetic properties of the nanostructured In-Ag alloy confined within a porous glass. The alloy composition was close to the eutectic point in the indium-rich range of the phase diagram. Temperature dependences of DC magnetization evidenced two superconducting transitions at 4.05 and 3.38 K. The magnetization isotherms demonstrated the superposition of two hysteresis loops with low and high critical fields below the second transition, a single hysteresis between the transitions and ferromagnetism with weak remanence in the normal state of the alloy. The shape of the loop seen below the second transition, which closes at a low magnetic field, corresponded to the intermediate state of the type-I superconductor. It was ascribed to strongly linked indium segregates. The loop observed below the first transition is referred to as type-II superconductivity. The secondary and tertiary magnetization branches measured at decreasing and increasing fields were shifted relative to each other, revealing the proximity of superconducting and ferromagnetic phases at the nanometer scale. This phenomenon was observed for the first time in the alloy, whose components were not magnetic in bulk. The sign of the shift shows the dominant role of the stray fields of ferromagnetic regions. Ferromagnetism was suggested to emerge at the interface between the In and AgIn2 segregates.

Orchestration of ferro- and anti-ferromagnetic ordering in gold nanoclusters

The unpaired electron in the gold clusters (Aun, n = no. of Au atoms) with an odd number of total electrons is solely responsible for the magnetic properties in the small-sized Au nano-clusters. However, no such unpaired electron is available due to pairing in the even number of atom gold clusters and behaving as a diamagnetic entity similar to bulk gold. In this work, we unveiled the spin-density distribution of odd Aun clusters with n = 1 to 19 that reveals that a single unpaired electron gets distributed non-uniformly among all Au-atoms depending on the cluster size and morphology. The delocalization of the unpaired electron leads to the spin dilution approaching a value of ∼1/n spin moments on each atom for the higher clusters. Interestingly, small odd-numbered gold clusters possess spin-magnetic moments similar to the delocalized spin moments as of organic radicals. Can cooperative magnetic properties be obtained by coupling these individual magnetic gold nanoparticles? In this work, by applying state-of-the-art computational methodologies, we have demonstrated ferromagnetic or anti-ferromagnetic couplings between such magnetic nanoclusters upon designing suitable organic spacers. These findings will open up a new avenue of nanoscale magnetic materials combining organic spacers and odd-electron nano-clusters.

Theoretical search for characteristic atoms in supported gold nanoparticles: a large-scale DFT study

The size and site dependences of atomic and electronic structures in isolated and supported gold nanoparticles have been investigated using large-scale density functional theory (DFT) calculations using multi-site support functions. The effects of the substrate on nanoparticles with diameters of 2 nm and several different shapes have been examined. First, isolated gold nanoparticles with diameters of 0.6 nm (13 atoms) to 4.5 nm (2057 atoms), which have comparable sizes to nanoparticles used in experiments, were considered. To analyse huge amounts of data obtained from large-scale DFT calculations, we performed principal component analysis (PCA), which helps systematically and efficiently clarify the electronic structures of large nanoparticles. The PCA results reveal the site dependence of the electronic structures. Notably, the atoms in the surface and subsurface have different electronic structures to those located in the inner layers, especially at the vertexes of the particles. The convergence of local electronic structures with respect to the particle size has also been demonstrated. For supported nanoparticles, PCA helps indicate which atoms are affected, and how much, by the substrate. The correlation between the PCA results and site dependence of reaction activity is also discussed herein.

Hybrid mesoporous electrodes evidence CISS effect on water oxidation

Controlling product selectivity is essential for improving the efficiency of multi-product reactions. Electrochemical water oxidation is a reaction of main importance in different applications, e.g., renewable energy schemes and environmental protection, where H2O2 and O2 are the two principal products. In this Communication, the product selectivity of electrochemical water oxidation was controlled by making use of the chiral induced spin selectivity (CISS) effect at mesoporous-TiO2 on the molecule-modified Au substrate. Our results show a decrease in H2O2 formation when using chiral hetero-helicene molecules adsorbed on the Au substrate. We propose a mechanism for this kinetic effect based on the onset of CISS-induced spin polarization on the Au-helicene chiral interface. We also present a new tunable substrate to investigate the CISS mechanism.

Magnetic control over the fundamental structure of atomic wires

When reducing the size of materials towards the nanoscale, magnetic properties can emerge due to structural variations. Here, we show the reverse effect, where the structure of nanomaterials is controlled by magnetic manipulations. Using the break-junction technique, we find that the interatomic distance in platinum atomic wires is shorter or longer by up to ∼20%, when a magnetic field is applied parallel or perpendicular to the wires during their formation, respectively. The magnetic field direction also affects the wire length, where longer (shorter) wires are formed under a parallel (perpendicular) field. Our experimental analysis, supported by calculations, indicates that the direction of the applied magnetic field promotes the formation of suspended atomic wires with a specific magnetization orientation associated with typical orbital characteristics, interatomic distance, and stability. A similar effect is found for various metal and metal-oxide atomic wires, demonstrating that magnetic fields can control the atomistic structure of different nanomaterials when applied during their formation stage.

Magnetic effects can emerge due to structural variations when the size of materials is reduced towards the nanoscale. Here, Chakrabarti et al demonstrates the opposite effect, showing that the interatomic distance in atomic wires changes by up to 20% depending on the orientation of an applied magnetic field.

Evidence of Au(II) and Au(0) States in Bovine Serum Albumin-Au Nanoclusters Revealed by CW-EPR/LEPR and Peculiarities in HR-TEM/STEM Imaging

Bovine serum albumin-embedded Au nanoclusters (BSA-AuNCs) are thoroughly probed by continuous wave electron paramagnetic resonance (CW-EPR), light-induced EPR (LEPR), and sequences of microscopic investigations performed via high-resolution transmission electron microscopy (HR-TEM), scanning transmission electron microscopy (STEM), and energy dispersive X-ray analysis (EDS). To the best of our knowledge, this is the first report analyzing the BSA-AuNCs by CW-EPR/LEPR technique. Besides the presence of Au(0) and Au(I) oxidation states in BSA-AuNCs, the authors observe a significant amount of Au(II), which may result from a disproportionation event occurring within NCs: 2Au(I) → Au(II) + Au(0). Based on the LEPR experiments, and by comparing the behavior of BSA versus BSA-AuNCs under UV light irradiation (at 325 nm) during light off-on-off cycles, any energy and/or charge transfer event occurring between BSA and AuNCs during photoexcitation can be excluded. According to CW-EPR results, the Au nano assemblies within BSA-AuNCs are estimated to contain 6–8 Au units per fluorescent cluster. Direct observation of BSA-AuNCs by STEM and HR-TEM techniques confirms the presence of such diameters of gold nanoclusters in BSA-AuNCs. Moreover, in situ formation and migration of Au nanostructures are observed and evidenced after application of either a focused electron beam from HR-TEM, or an X-ray from EDS experiments.

Observation of Cu Spin Fluctuations in High-Tc Cuprate Superconductor Nanoparticles Investigated by Muon Spin Relaxation

The nano-size effects of high-T_c cuprate superconductor La_2-xSr_xCuO₄ with x = 0.20 are investigated using X-ray diffractometry, Transmission electron microscopy, and muon-spin relaxation (μSR). It is investigated whether an increase in the bond distance of Cu and O atoms in the conducting layer compared to those of the bulk state might affect its physical and magnetic properties. The μSR measurements revealed the slowing down of Cu spin fluctuations in La_2-xSr_xCuO₄ nanoparticles, indicating the development of a magnetic correlation at low temperatures. The magnetic correlation strengthens as the particle size reduces. This significantly differs from those observed in the bulk form, which show a superconducting state below T_c. It is indicated that reducing the particle size of La_2-xSr_xCuO₄ down to nanometer size causes the appearance of magnetism. The magnetism enhances with decreasing particle size.

Magnetism of Dendrimer-Coated Gold Nanoparticles: A Size and Functionalization Study

Highly sensitive magnetometry reveals paramagnetism in dendrimer-coated gold nanoparticles. Different types of such nanoparticles, as a result of (i) functionalizing with two distinct Percec-type dendrons, linked to gold via dodecanethiol groups, and (ii) postsynthesis annealing in a solvent-free environment that further promotes their growth have been prepared. Ultimately, for each of the two functionalization configurations, we obtain highly monodisperse and stable nanoparticles of two different sizes, with spherical shape. These characteristics allow singling out the source of the measured paramagnetic signals as exclusively arising from the undercoordinated gold atoms on the surfaces of the nanoparticles. Bulk gold and the functional groups of the ligands contribute only diamagnetically.

A comparative study of structural, electronic, and optical properties of thiolated gold clusters with icosahedral vs face-centered cubic cores

The structural, electronic, and optical properties of the protected Au clusters with icosahedral (Ih) and face-centered cubic (FCC)-like Au₁₃ cores were studied to understand the origin of the difference in the optical gaps of these clusters. It has been demonstrated that the choice of density functionals does not qualitatively affect the properties of Au₂₃ and Au₂₅ clusters with Ih and FCC cores. The density of states, molecular orbitals, and natural charges were analyzed in detail using the B3LYP functional. The substantial energy difference in the lowest-energy absorption peaks for the clusters with the Ih and FCC cores is attributed to the difference in the natural charges of the central Au atoms (Au_c) in the Ih and FCC cores, the former of which is more negative than the latter. Natural population analysis demonstrates that the excess negative charge of the Au_c atom in clusters with Ih cores occupies the 6p atomic orbitals. This difference in Au_c is attributed to the smaller size of the Ih core compared to the FCC core, as a less bulky ligand allows a smaller core with increased electron density, which, in turn, increases the highest occupied molecular orbital energy and decreases the optical gap.

Single Mn Atom Doping in Chiral Sensitive Assembled Gold Clusters to Molecular Magnet

Chiral stirred optical and magnetic properties, through the doping of assembled ultrasmall metal clusters (AMCs), are promising discernment to rivet the molecule-like quantum devices. Here, the single manganese (Mn) atom doping and assembly of the gold cluster (Au₈), leading to the chirality driven magnetism, has been achieved through a ligand-mediated growth. The X-ray absorption near edge structure and electron paramagnetic resonance studies corroborate the tetrahedral coordinated local structure of Mn dopant in the Au host. The optical and vibrational circular dichroic analysis affirms the modulation of chirality (negative to positive) in the presence of the Mn. A distinct ferromagnetic hysteresis loop at 300 K shows Mn ridden chiral sensitive ferromagnetism in contrary to the ligand influenced superparamagnetic undoped AMCs. The spin-polarized density functional theory level of calculations reveal the partial overlapping of spin-up and -down density of states in the doped AMCs, attributing to the ferromagnetic nature as like a molecular magnet suitable for the opto-spintronics application.

In Situ Small-Angle X-ray Scattering Studies on the Growth Mechanism of Anisotropic Platinum Nanoparticles

Shape-controlled platinum nanoparticles exhibit extremely high oxygen reduction activity. Platinum nanoparticles were synthesized by the reduction of a platinum complex in the presence of a soft template formed by organic surfactants in oleylamine. The formation of platinum nanoparticles was investigated using in situ small-angle X-ray scattering experiments. Time-resolved measurements revealed that different particle shapes appeared during the reaction. After the nuclei were generated, they grew into anisotropic rod-shaped nanoparticles. The shape, size, number density, reaction yield, and specific surface area of the nanoparticles were successfully determined using small-angle X-ray scattering profiles. Anisotropic platinum nanoparticles appeared at a low reaction temperature (∼100 °C) after a short reaction time (∼30 min). The aspect ratio of these platinum nanoparticles was correlated with the local packing motifs of the surfactant molecules and their stability. Our findings suggest that the interfacial structure between the surfactant and platinum nuclei can be important as a controlling factor for tailoring the aspect ratio of platinum nanoparticles and further optimizing the fuel cell performance.

Tuning the magnetism of gold nanoparticles by changing the thiol coating

Room-temperature ferromagnetic behaviour has been reported in nanoscale materials expected to be diamagnetic, including gold. However, it is yet unclear which factors (size, shape, surface coating) predominantly influence the magnitude of the magnetic response. In this work, we study the magnetic and electronic properties of similarly-sized gold nanoparticles (Au NPs) coated with four different n-alkanethiols, as well as hydroxyl- and carboxyl-functionalized alkanethiols using superconducting quantum interference device (SQUID) magnetometry and ultraviolet photoelectron spectroscopy (UPS). We find room-temperature behaviour (hysteresis in magnetization vs. field strength loops) in all samples, as well as large effective magnetic anisotropy. Importantly, we find the nanoparticles coated with polar chain end-groups (-OH and -COOH) show markedly higher magnetization; this increased magnetization correlates with a higher work function. This work establishes chemical handles to enhance magnetism in nanoscale gold particles.

Noble Metal Nanoparticles in Pectin Matrix. Preparation, Film Formation, Property Analysis, and Application in Electrocatalysis

Stable polymeric materials with embedded nano-objects, retaining their specific properties, are indispensable for the development of nanotechnology. Here, a method to obtain Pt, Pd, Au, and Ag nanoparticles (ca. 10 nm, independent of the metal) by the reduction of their ions in pectin, in the absence of additional reducing agents, is described. Specific interactions between the pectin functional groups and nanoparticles were detected, and they depend on the metal. Bundles and protruding nanoparticles are present on the surface of nanoparticles/pectin films. These films, deposited on the electrode surface, exhibit electrochemical response, characteristic for a given metal. Their electrocatalytic activity toward the oxidation of a few exemplary organic molecules was demonstrated. In particular, a synergetic effect of simultaneously prepared Au and Pt nanoparticles in pectin films on glucose electro-oxidation was found.

Site-Dependent Spin Delocalization and Evidence of Ferrimagnetism in Atomically Precise Au25(SR)180 Clusters as Seen by Solution 13C NMR Spectroscopy

We report a simple but detailed solution ¹³C nuclear magnetic resonance spectroscopic study of atomically precise neutral Au₂₅(SR)₁₈⁰ (SR = alkyl thiolate) clusters. The paramagnetic ¹³C Knight shift of alkyl chain carbons, which is proportional to the local electron spin density, exhibits an electron spin delocalization that exponentially decays along the alkyl chain. The magnitude and decay constant of the observed electron spin delocalization, although largely independent of alkyl chain length, depend on where, that is, "in" versus "out" (vide infra) position, the alkyl chain is bound, in agreement with density functional theory calculations. Notably, the determined position-dependent decay constants, 1.70/Å and 0.41/Å for "in" and "out" ligands, respectively, not only could have important ramifications in molecular spintronics but are also comparable to measured decay constants in molecular electrical conductance of alkyl chains, potentially offering an alternative, simple method for estimating the latter. Moreover, the negative intercept temperatures of linear fits of reciprocal ¹³C (as well its bound ¹H) Knight shift versus temperature strongly suggest the existence of local ferrimagnetism in individual Au₂₅(SR)₁₈⁰ clusters.

Organic/metal interface-modulated magnetism in [Fe/C60]3 multilayers and Fe-C60 composites

Because of the expected long spin-transport length of organic materials, the magnetic metal/organic interface is crucial to the application of organic spintronics. In this study, [Fe/C₆₀]₃ multilayers were fabricated for the investigation of C₆₀-mediated magnetic interlayer coupling. [Fe/C₆₀]₃ thin films were characterized using the magneto-optical Kerr effect, transmission electron microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS). The thin films revealed in-plane magnetic anisotropy, and the magnetic coercivity (H _c ) drastically decreased from 6-8 mT to 0.5 mT with the increase of C₆₀ thickness from 0.1 nm to 5 nm. The insertion of the C₆₀ layer considerably reduced H _c because a thickness greater than 1 nm of the C₆₀ layer is sufficient for blocking magnetic exchange coupling between Fe layers. In addition, post-annealing increased H _c because of Fe inter-diffusion, which promotes magnetic exchange coupling and further Fe-C bonding, as confirmed by a comparative study of XPS C-spectra. The thermally triggered inter-diffusion between Fe and C₆₀ layers turned the multilayers into a mixed composite film and thus caused magnetic variation. Annealing time and temperature can be used as control parameters for the tuning of magnetism in Fe-C₆₀ composites.

Surface spin magnetism controls the polarized exciton emission from CdSe nanoplatelets

The surface of nominally diamagnetic colloidal CdSe nanoplatelets can demonstrate paramagnetic behaviour owing to the uncompensated spins of dangling bonds, as we reveal here by optical spectroscopy in high magnetic fields up to 15 T using the exciton spin as a probe of the surface magnetism. The strongly nonlinear magnetic field dependence of the circular polarization of the exciton emission is determined by the magnetization of the dangling-bond spins (DBSs), the exciton spin polarization as well as the spin-dependent recombination of dark excitons. The sign of the exciton-DBS exchange interaction depends on the nanoplatelet growth conditions.

The di(thiourea)gold(I) complex [Au{S=C(NH2)2}2][SO3Me] as a precursor for the convenient preparation of gold nanoparticles

The synthesis of [Au{S=C(NH2)2}2][SO3Me] (1) (a) by the anodic oxidation of gold metal in an anolyte of thiourea and methansulfonic acid and (b) by the reaction of Au(OH)3 with an aqueous solution of methanesulfonic acid in the presence of thiourea is reported. The structure of 1 in the solid state has been determined by single-crystal X-ray diffraction showing a linear S–Au–S unit with the thiourea ligands in a leaflet structure folded by 113.2(3)°. The cation of complex 1 is a dimer, based on short S · · · C interactions between two adjacent mononuclear cations. The thermal decomposition behavior of 1 was studied by TG and TG-MS confirming that it decomposes under inert gas or oxygen atmosphere in four steps in the temperature range of 200–650°C. Initial decomposition starts with the release and fragmentation of one of the thiourea ligands, followed by the anion degradation. Powder X-ray diffraction studies specified the formation of gold metal. Based on this observation, complex 1 was used as precursor for the formation of gold nanoparticles (Au NPs) in 1-hexadecylamine (c = 4.0 mol L−1) at T = 330°C without any addition of reducing agents. TEM, electron diffraction, and UV/Vis spectroscopy studies were carried out. Au NPs of size 15 ± 4 nm were formed, showing the characteristic surface plasmon resonance at 528 nm.

Paramagnetic gold in a highly disordered Au-Ni-O alloy

Magnetic materials are usually classified into a distinct category such as diamagnets, paramagnets or ferromagnets. The enormous progress in materials science allows one nowadays, however, to change the magnetic nature of an element in a material. Gold, in bulk form, is traditionally a diamagnet. But in a ferromagnetic environment, it can adopt an induced ferromagnetic moment. Moreover, the growth of gold under certain conditions may lead to a spontaneous ferromagnetic or paramagnetic response. Here, we report on paramagnetic gold in a highly disordered Au-Ni-O alloy and focus on the unusual magnetic response. Such materials are mainly considered for plasmonic applications. Thin films containing Au, Ni and NiO are fabricated by co-deposition of Ni and Au in a medium vacuum of 2 × 10^-2 mbar. As a result, Au is in a fully disordered state forming in some cases isolated nanocrystallites of up to 4 nm in diameter as revealed by high resolution transmission electron microscopy. The disorder and the environment, which is rich in oxygen, lead to remarkable magnetic properties of Au: an induced ferromagnetic and a paramagnetic state. This can be proven by measuring the x-ray magnetic circular dichroism. Our experiments show a way to establish and monitor Au paramagnetism in alloys.

Single Domain 10 nm Ferromagnetism Imprinted on Superparamagnetic Nanoparticles Using Chiral Molecules

The rapid growth in demand for data and the emerging applications of Big Data require the increase of memory capacity. Magnetic memory devices are among the leading technologies for meeting this demand; however, they rely on the use of ferromagnets that creates size reduction limitations and poses complex materials requirements. Usually magnetic memory sizes are limited to 30-50 nm. Reducing the size even further, to the ≈10-20 nm scale, destabilizes the magnetization and its magnetic orientation becomes susceptible to thermal fluctuations and stray magnetic fields. In the present work, it is shown that 10 nm single domain ferromagnetism can be achieved. Using asymmetric adsorption of chiral molecules, superparamagnetic iron oxide nanoparticles become ferromagnetic with an average coercive field of ≈80 Oe. The asymmetric adsorption of molecules stabilizes the magnetization direction at room temperature and the orientation is found to depend on the handedness of the chiral molecules. These studies point to a novel method for the miniaturization of ferromagnets (down to ≈10 nm) using established synthetic protocols.

A perspective on the magnetism of alkanethiol-coated gold thin films

Intriguing ferromagnetic behaviour has been reported in gold thin films — a diamagnetic material in the bulk — wherein large magnetic moments and uncommon anisotropy are often hallmark features. The tuning of the electronic and magnetic properties by the presence of molecular self-assembled monolayers has been proposed. In this work, we present the study of the magnetism of a wide collection of alkanethiols of differing chain lengths coated on Au. We find no or only very weak magnetism, casting doubt on the universality and reproducibility of this phenomenon.

Green synthesis of bio-molecule encapsulated magnetic silver nanoparticles and their antibacterial activity

Persuaded by the necessity of finding new sources of antibiotics, silver nanoparticles (Ag NPs) were synthesized by adopting a newly developed green synthesis technique and subsequently, their antibacterial activity against different pathogenic bacteria was evaluated. We have successfully synthesized bio-molecule capped ferromagnetic Ag NPs with an average crystallite size of 13 nm using AgNO3 solution as a precursor and Artocarpus heterophyllus leaf extract as a reducing and capping agent. The characterization of the synthesized Ag NPs was carried out using various techniques such as UV-visible (UV-Vis) spectroscopy, energy dispersive X-ray (EDX) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), thermogravimetry (TG), and vibrating sample magnetometer (VSM) analyses. After exposing the synthesized Ag NPs to two Gram-positive bacteria - Staphylococcus aureus and Bacillus cereus and two Gram-negative bacteria - Escherichia coli and Salmonella typhimurium, the zones of inhibition were found to be 15, 16, 19, and 18 mm, respectively. These results imply that the Artocarpus heterophyllus leaf extract mediated green synthesized bio-molecules encapsulated Ag NPs can be considered as a potential antibiotic against human pathogens which is very encouraging.

Single Nanoparticle Magnetic Spin Memristor

There is an increasing demand for the development of a simple Si-based universal memory device at the nanoscale that operates at high frequencies. Spin-electronics (spintronics) can, in principle, increase the efficiency of devices and allow them to operate at high frequencies. A primary challenge for reducing the dimensions of spintronic devices is the requirement for high spin currents. To overcome this problem, a new approach is presented that uses helical chiral molecules exhibiting spin-selective electron transport, which is called the chiral-induced spin selectivity (CISS) effect. Using the CISS effect, the active memory device is miniaturized for the first time from the micrometer scale to 30 nm in size, and this device presents memristor-like nonlinear logic operation at low voltages under ambient conditions and room temperature. A single nanoparticle, along with Au contacts and chiral molecules, is sufficient to function as a memory device. A single ferromagnetic nanoplatelet is used as a fixed hard magnet combined with Au contacts in which the gold contacts act as soft magnets due to the adsorbed chiral molecules.

Photochromism into nanosystems: towards lighting up the future nanoworld

The ability to manipulate the structure and function of promising nanosystems via energy input and external stimuli is emerging as an attractive paradigm for developing reconfigurable and programmable nanomaterials and multifunctional devices. Light stimulus manifestly represents a preferred external physical and chemical tool for in situ remote command of the functional attributes of nanomaterials and nanosystems due to its unique advantages of high spatial and temporal resolution and digital controllability. Photochromic moieties are known to undergo reversible photochemical transformations between different states with distinct properties, which have been extensively introduced into various functional nanosystems such as nanomachines, nanoparticles, nanoelectronics, supramolecular nanoassemblies, and biological nanosystems. The integration of photochromism into these nanosystems has endowed the resultant nanostructures or advanced materials with intriguing photoresponsive behaviors and more sophisticated functions. In this Review, we provide an account of the recent advancements in reversible photocontrol of the structures and functions of photochromic nanosystems and their applications. The important design concepts of such truly advanced materials are discussed, their fabrication methods are emphasized, and their applications are highlighted. The Review is concluded by briefly outlining the challenges that need to be addressed and the opportunities that can be tapped into. We hope that the review of the flourishing and vibrant topic with myriad possibilities would shine light on exploring the future nanoworld by encouraging and opening the windows to meaningful multidisciplinary cooperation of engineers from different backgrounds and scientists from the fields such as chemistry, physics, engineering, biology, nanotechnology and materials science.

Emergent magnetism at transition-metal-nanocarbon interfaces

Charge transfer at metallo-molecular interfaces may be used to design multifunctional hybrids with an emergent magnetization that may offer an eco-friendly and tunable alternative to conventional magnets and devices. Here, we investigate the origin of the magnetism arising at these interfaces by using different techniques to probe 3d and 5d metal films such as Sc, Mn, Cu, and Pt in contact with fullerenes and rf-sputtered carbon layers. These systems exhibit small anisotropy and coercivity together with a high Curie point. Low-energy muon spin spectroscopy in Cu and Sc-C60 multilayers show a quick spin depolarization and oscillations attributed to nonuniform local magnetic fields close to the metallo-carbon interface. The hybridization state of the carbon layers plays a crucial role, and we observe an increased magnetization as sp3 orbitals are annealed into sp2-π graphitic states in sputtered carbon/copper multilayers. X-ray magnetic circular dichroism (XMCD) measurements at the carbon K edge of C60 layers in contact with Sc films show spin polarization in the lowest unoccupied molecular orbital (LUMO) and higher π*-molecular levels, whereas the dichroism in the σ*-resonances is small or nonexistent. These results support the idea of an interaction mediated via charge transfer from the metal and dz-π hybridization. Thin-film carbon-based magnets may allow for the manipulation of spin ordering at metallic surfaces using electrooptical signals, with potential applications in computing, sensors, and other multifunctional magnetic devices.

Magnetic Nanoplatelet-Based Spin Memory Device Operating at Ambient Temperatures

There is an increasing demand for realizing a simple Si based universal memory device working at ambient temperatures. In principle, nonvolatile magnetic memory can operate at low power consumption and high frequencies. However, in order to compete with existing memory technology, size reduction and simplification of the used material systems are essential. In this work, the chiral-induced spin selectivity effect is used along with 30-50 nm ferromagnetic nanoplatelets in order to realize a simple magnetic memory device. The vertical memory is Si compatible, easy to fabricate, and in principle can be scaled down to a single nanoparticle size. Results show clear dual magnetization behavior with threefold enhancement between the one and zero states. The magnetization of the device is accompanied with large avalanche like noise that is ascribed to the redistribution of current densities due to spin accumulation inducing coupling effects between the different nanoplatelets.

Magnetization switching in ferromagnets by adsorbed chiral molecules without current or external magnetic field

Ferromagnets are commonly magnetized by either external magnetic fields or spin polarized currents. The manipulation of magnetization by spin-current occurs through the spin-transfer-torque effect, which is applied, for example, in modern magnetoresistive random access memory. However, the current density required for the spin-transfer torque is of the order of 1 × 106 A·cm-2, or about 1 × 1025 electrons s-1 cm-2. This relatively high current density significantly affects the devices' structure and performance. Here we demonstrate magnetization switching of ferromagnetic thin layers that is induced solely by adsorption of chiral molecules. In this case, about 1013 electrons per cm2 are sufficient to induce magnetization reversal. The direction of the magnetization depends on the handedness of the adsorbed chiral molecules. Local magnetization switching is achieved by adsorbing a chiral self-assembled molecular monolayer on a gold-coated ferromagnetic layer with perpendicular magnetic anisotropy. These results present a simple low-power magnetization mechanism when operating at ambient conditions.

Geometrical Effects on the Magnetic Properties of Nanoparticles

Elucidating the connection between shape and properties is a challenging but essential task for a rational design of nanoparticles at the atomic level. As a paradigmatic example we investigate how geometry can influence the magnetic properties of nanoparticles, focusing in particular on platinum clusters of 1-2 nm in size. Through first-principle calculations, we have found that the total magnetization depends strongly on the local atomic arrangements. This is due to a contraction of the nearest neighbor distance together with an elongation of the second nearest neighbor distance, resulting in an interatomic partial charge transfer from the atoms lying on the subsurface layer (donors) toward the vertexes (acceptors).

Growth behavior of gold nanoparticles synthesized in unsaturated fatty acids by vacuum evaporation methods

Physical vapor evaporation of metals on low vapor pressure liquids is a simple and clean method to synthesize nanoparticles and thin films, though only little work has been conducted so far. Here, gold nanoparticles were synthesized by vacuum evaporation (VE) methods in ricinoleic acid and oleic acid, two typical unsaturated fatty acids (UFAs). The two solvents formed black aggregates after deposition and then shrunk and finally disappeared with the progress of time. By transmission electron microscopy (TEM) images, nanoparticles in ricinoleic acids formed aggregates and then dispersed by time, while in oleic acid big aggregates were not observed in all timescales. From TEM images and small angle X-ray scattering (SAXS) measurements, the mean size of the nanoparticles was about 4 nm in both ricinoleic and oleic acids. UV-Vis spectra were also taken as a function of time and the results were consistent with the growth behavior presumed by TEM images. Air exposure had an influence on the behavior of the sample triggering the nanoparticle formation in both solvents. From control experiments, we discovered that oxygen gas triggered the phenomenon and nanoparticles function as a catalyst for the oxidation of the UFAs. It stimulates the phenomenon and in ricinoleic acid, specifically, electrons are transferred from riconleic acid to the gold nanoparticles, enhancing the surface potential of the nanoparticles and the repulsive force between their electronic double layers.

Unusual ferromagnetic behaviour of embedded non-functionalized Au nanoparticles in Bi/Au bilayer films

Observation of ferromagnetic behavior of non-functionalized gold nanoparticles in contrast to the diamagnetic nature of bulk gold synthesized by single step Au ion irradiation.

A high-spin organic diradical as a spin filter

Here, in this work we have designed a molecular bridge structure which can be used as a spin filter where the prototypical highly ferromagnetic m-phenylene connected bis(aminoxyl) diradical is used as a bridging fragment between two semi-infinitely widened gold (Au) electrodes along the [100] direction. A state-of-the-art non-equilibrium Green function's (NEGF) method coupled with the density functional theory (DFT) was carried out on this two-probe molecular bridge system to understand its electrical spin transport characteristics. The spin current at various bias voltages from 0.00 V to 4.00 V at intervals of 0.20 V for this Au-diradical-Au molecular junction is evaluated. We also quantify the bias-dependent spin injection coefficients (BDSIC) at different bias voltages and also the spin-filter efficiency at equilibrium, i.e., at zero bias voltage. Also plots of BDSIC vs. voltage, the up- and down-spin current vs. voltage (I-V) curves, and density of states (DOS) at zero bias voltage are evaluated.

Raman modes, dipole moment and chirality in periodically positioned Au8clusters

The solution-based assembly of h-Au₈clusters into a local periodic structure (LPS) of periodicity 1.47 nm with negative chirality as confirmed by experimental and theoretical analyses exhibits excitonic couplet state.

Self-assembly of NiTPP on Cu(111): a transition from disordered 1D wires to 2D chiral domains

The growth mechanism of NiTPP on Cu(111): from disordered 1D wires to self-assembled chiral domains.

EXAFS and XANES analysis of oxides at the nanoscale

Worldwide research activity at the nanoscale is triggering the appearance of new, and frequently surprising, materials properties in which the increasing importance of surface and interface effects plays a fundamental role. This opens further possibilities in the development of new multifunctional materials with tuned physical properties that do not arise together at the bulk scale. Unfortunately, the standard methods currently available for solving the atomic structure of bulk crystals fail for nanomaterials due to nanoscale effects (very small crystallite sizes, large surface-to-volume ratio, near-surface relaxation, local lattice distortions etc.). As a consequence, a critical reexamination of the available local-structure characterization methods is needed. This work discusses the real possibilities and limits of X-ray absorption spectroscopy (XAS) analysis at the nanoscale. To this end, the present state of the art for the interpretation of extended X-ray absorption fine structure (EXAFS) is described, including an advanced approach based on the use of classical molecular dynamics and its application to nickel oxide nanoparticles. The limits and possibilities of X-ray absorption near-edge spectroscopy (XANES) to determine several effects associated with the nanocrystalline nature of materials are discussed in connection with the development of ZnO-based dilute magnetic semiconductors (DMSs) and iron oxide nanoparticles.

Observation of the origin of d0 magnetism in ZnO nanostructures using X-ray-based microscopic and spectroscopic techniques

Efforts have been made to elucidate the origin of d(0) magnetism in ZnO nanocactuses (NCs) and nanowires (NWs) using X-ray-based microscopic and spectroscopic techniques. The photoluminescence and O K-edge and Zn L3,2-edge X-ray-excited optical luminescence spectra showed that ZnO NCs contain more defects than NWs do and that in ZnO NCs, more defects are present at the O sites than at the Zn sites. Specifically, the results of O K-edge scanning transmission X-ray microscopy (STXM) and the corresponding X-ray-absorption near-edge structure (XANES) spectroscopy demonstrated that the impurity (non-stoichiometric) region in ZnO NCs contains a greater defect population than the thick region. The intensity of O K-edge STXM-XANES in the impurity region is more predominant in ZnO NCs than in NWs. The increase in the unoccupied (occupied) density of states at/above (at/below) the conduction-band minimum (valence-band maximum) or the Fermi level is related to the population of defects at the O sites, as revealed by comparing the ZnO NCs to the NWs. The results of O K-edge and Zn L3,2-edge X-ray magnetic circular dichroism demonstrated that the origin of magnetization is attributable to the O 2p orbitals rather than the Zn d orbitals. Further, the local density approximation (LDA) + U verified that vacancies in the form of dangling or unpaired 2p states (due to Zn vacancies) induced a significant local spin moment in the nearest-neighboring O atoms to the defect center, which was determined from the uneven local spin density by analyzing the partial density of states of O 2p in ZnO.

Large orbital magnetic moment in Pt₁₃ clusters

We present an extensive study of Pt₁₃ clusters embedded in a Na-Y zeolite, by comparing calculations for isolated clusters to experimental data. We perform structural refinements for various geometries involving the isolated clusters and calculate the corresponding x-ray absorption and magnetic circular dichroism spectra, from the joint perspective of pseudopotential plane wave calculations and real space multiple scattering theory. Taking into account the spin-orbit coupling significantly improves the previous scalar relativistic predictions of magnetic properties. The ensemble of embedded Pt₁₃ is found to be dominated by a non-magnetic cuboctahedral geometry. One of the implications is that the ground state of Pt₁₃ clusters in the zeolite environment is different from that of isolated particles. We investigate several isomers that yield a magnetic signature. Furthermore, their abundance was estimated by direct comparison with experiment. We found that one third of the magnetic moment of Pt₁₃ comes from the orbital contribution, in agreement with the experimental value. We therefore provide theoretical proof of the extraordinary orbital magnetization in Pt13 clusters.