Optical bio/chemical sensors for vitamin B₁₂ analysis in food and pharmaceuticals: state of the art, challenges, and future outlooks

• Seyed Mohammad Taghi Gharibzahedi and
• Zeynep Altintas

Beilstein J. Nanotechnol. 2025, 16, 2207–2244, doi:10.3762/bjnano.16.153

Electron transport through nanoscale multilayer graphene and hexagonal boron nitride junctions

• Aleksandar Staykov and
• Takaya Fujisaki

Beilstein J. Nanotechnol. 2025, 16, 2132–2143, doi:10.3762/bjnano.16.147

• and Stone–Wales defects. Nitrogen doping transforms graphene from a zero-bandgap semiconductor to a metal, while Stone–Wales defects open the bandgap. For h-BN, we considered Stone–Wales defects. A detailed comparison of electron transport through five materials, that is, multilayer nanoscale graphene

• [2], nanoscale electronics and electronic components [3], thermoelectric devices [4], and transparent films [5]. Graphene is a two-dimensional zero-bandgap semiconductor with excellent bulk conductivity. Its in-plane electron transport strongly depends on lattice order, lattice defects, and three

• sites and hinders the electron mobility [7]. In addition, zigzag and armchair edges in graphene nanoribbons will open the bandgap and create trapping sites. Thus, small graphene nanoribbons will show higher electrical resistance compared to larger nanoribbons and infinite two-dimensional sheets. The in

Further insights into the thermodynamics of linear carbon chains for temperatures ranging from 13 to 300 K

• Alexandre Rocha Paschoal,
• Thiago Alves de Moura,
• Juan S. Rodríguez-Hernández,
• Carlos William de Araujo Paschoal,
• Yoong Ahm Kim,
• Morinobu Endo and
• Paulo T. Araujo

Beilstein J. Nanotechnol. 2025, 16, 1818–1825, doi:10.3762/bjnano.16.125

• with distinct N (LCC have their bandgap proportional to N−1; the smaller the chain, the larger the bandgap). Figure 2 corroborates this claim: ωLCC for similar LCC possess similar dependence on T. As previously discussed, ωLCC associated with each identified LCC is used as a probe to obtain the

Electrical, photocatalytic, and sensory properties of graphene oxide and polyimide implanted with low- and medium-energy silver ions

• Josef Novák,
• Eva Štěpanovská,
• Petr Malinský,
• Vlastimil Mazánek,
• Jan Luxa,
• Ulrich Kentsch and
• Zdeněk Sofer

Beilstein J. Nanotechnol. 2025, 16, 1794–1811, doi:10.3762/bjnano.16.123

• bandgap and an increased density of states near the Fermi level, promotes absorption in the visible region and facilitates the generation of electron–hole (e−–h+) pairs upon light irradiation. The silver clusters also exhibit plasmonic resonances, which amplify the local electromagnetic field and promote

• the generation of excited states with longer lifetimes [18]. Photocatalysis is a surface-driven process in which the absorption of photons with energy equal to or greater than the material’s bandgap results in the generation of e−–h+ pairs. These photogenerated charge carriers migrate to the surface

• semiconducting properties of the polymer matrix and reduces the bandgap width. Following the PI implantation with the lowest fluence, the Ag ions have the ability to bond with the newly created free bonds (C–C), resulting in the formation of oxides [21]. This process can lead to a reduction in the bandgap and

Nanotechnology-based approaches for the removal of microplastics from wastewater: a comprehensive review

• Nayanathara O Sanjeev,
• Manjunath Singanodi Vallabha and
• Rebekah Rubidha Lisha Rabi

Beilstein J. Nanotechnol. 2025, 16, 1607–1632, doi:10.3762/bjnano.16.114

• . Nanoparticles, owing to their high surface-to-volume ratio, demonstrate superior catalytic performance compared to bulk materials. Furthermore, the particle size of semiconductors influences their bandgap energy and crystalline structure, which in turn affects their redox potential and the spatial distribution

Photocatalytic degradation of ofloxacin in water assisted by TiO₂ nanowires on carbon cloth: contributions of H₂O₂ addition and substrate absorbability

• Iram Hussain,
• Lisha Zhang,
• Zhizhen Ye and
• Jin-Ming Wu

Beilstein J. Nanotechnol. 2025, 16, 1567–1579, doi:10.3762/bjnano.16.111

• and CC/NW-450 °C. The black carbon cloth absorbs light in the wavelength range of 250–800 nm, and the UV adsorption edge can be seen clearly after TiO2 precipitations. According to the Kubelka–Munk formula, assuming an indirect transition between valence and conduction bands [24], the bandgap for the

• CC/NW-450 °C is evaluated to be 2.97 eV (Figure 3a inset). This value is smaller than the bandgap of 3.2 eV for anatase TiO2, which can be attributed to the strong interaction between TiO2 and the carbon cloth, which may induce localized states within the bandgap and potentially introduce defects

• ) XRD patterns and (b) Raman spectra of hydrogen titanate nanowires (CC/HTNW) and TiO2 nanowires (CC/NW-450 °C) grown on carbon cloth. (a) UV–vis diffuse absorbance spectrum of the CC/NW-450 °C. The inset shows the replotting of (a) in (αhν)1/2 ~ hν coordinates to evaluate the bandgap of CC/NW-450 °C

Transient electronics for sustainability: Emerging technologies and future directions

• Jae-Young Bae,
• Myung-Kyun Choi and
• Seung-Kyun Kang

Beilstein J. Nanotechnol. 2025, 16, 1545–1556, doi:10.3762/bjnano.16.109

• circuits, such as ring oscillators, entirely composed of water-soluble materials, supporting its applicability in future bioelectronic devices. Nevertheless, the currently known repertoire of bioresorbable semiconductors remains narrow, both in material selection and bandgap range. Expanding the library to

• include materials with diverse bandgap properties remains a key challenge as it would enable wavelength-specific and electrically optimized device designs across a wide array of applications, including sensors, radio frequency (RF) devices, energy harvesters, and optoelectronic systems. For instance, low

• -bandgap bioresorbable semiconductors based on magnesium–silicon alloys, such as Mg2Si, could be promising candidates to fill this gap (Figure 2a) [49][50]. However, further exploration is needed to discover and engineer additional semiconducting materials, including a broader range of silicon- or

Influence of laser beam profile on morphology and optical properties of silicon nanoparticles formed by laser ablation in liquid

• Natalie Tarasenka,
• Vladislav Kornev,
• Alena Nevar and
• Nikolai Tarasenko

Beilstein J. Nanotechnol. 2025, 16, 1533–1544, doi:10.3762/bjnano.16.108

• , provides the information about the bandgap energies of the prepared Si NPs. Here, α, h, and ν are absorption coefficient, Planck constant, and frequency, respectively; n depends on the type of bandgap and can be 2 for indirect or 1/2 for direct bandgaps, respectively (Figure 6b). For silicon, indirect

• bandgap characteristics are typical; therefore, the value of n was chosen as 2. Note that silicon is known to have different crystal structures, whose optical properties may vary. Furthermore, size effects are also influencing the optical properties of the resulting NPs [42]. As shown in [42], the bandgap

• of nanocrystals increases with decreasing size due to quantum confinement. Accordingly, the estimated bandgaps for all the prepared colloids was found to be in the range of 1.8–2.2 eV, which is larger than the bandgap of bulk Si (1.12 eV) and can be attributed to the NPs size effects (Figure 6b). The

Laser processing in liquids: insights into nanocolloid generation and thin film integration for energy, photonic, and sensing applications

• Akshana Parameswaran Sreekala,
• Pooja Raveendran Nair,
• Jithin Kundalam Kadavath,
• Bindu Krishnan,
• David Avellaneda Avellaneda,
• M. R. Anantharaman and
• Sadasivan Shaji

Beilstein J. Nanotechnol. 2025, 16, 1428–1498, doi:10.3762/bjnano.16.104

• characteristics, are influenced by the NP size and crystalline quality (Figure 3b,c). LFL also plays a crucial role in tuning the optical properties of NPs. Laser processing affects the defect density and optical bandgap of the particles, as demonstrated by changes in the optical transmission spectra before and

• , which in turn altered the optical properties of the films. This resulted in a decrease in the optical bandgap (1.5 eV) compared to the NPs in colloidal form [129]. AFM images acquired from MoO3 NPs film confirmed the uniform distribution of the spherical particles onto the substrate. A variable grain

Enhancing the photoelectrochemical performance of BiOI-derived BiVO₄ films by controlled-intensity current electrodeposition

• Huu Phuc Dang,
• Khanh Quang Nguyen,
• Nguyen Thi Mai Tho and
• Tran Le

Beilstein J. Nanotechnol. 2025, 16, 1289–1301, doi:10.3762/bjnano.16.94

• absorption, moderate bandgap (≈2.4 eV), high theoretical photocurrent density (≈7.5 mA·cm−2), and chemical stability in aqueous environments [7][8][9]. Nevertheless, BiVO4 suffers from intrinsic drawbacks such as low charge carrier mobility, limited conductivity, and rapid recombination of photogenerated

• ). Bandgap values were determined using Tauc plots for indirect allowed transitions, based on (αhν)2 ∝ (hν – Eg), where α is the absorption coefficient, h is Planck’s constant, ν is the frequency, and Eg is the bandgap energy. The (αhν)2 values plotted against the photon energy determined Eg at the

• improvements through stronger peaks, suggesting fewer defects. The decrease in the bandgap (≈0.16 eV) is consistent with research linking oxygen vacancies to band tailing in BiVO4 films [26]. Besides, Figure 3a shows that the BiVO4(326) and BiVO4(324) samples have absorption that goes beyond 520 nm, with some

Electronic and optical properties of chloropicrin adsorbed ZnS nanotubes: first principle analysis

• Prakash Yadav,
• Boddepalli SanthiBhushan and
• Anurag Srivastava

Beilstein J. Nanotechnol. 2025, 16, 1184–1196, doi:10.3762/bjnano.16.87

• , optical absorption, and optical conductivity of the ZnS NT-CP system. Our findings reveal that the interaction between CP and ZnS NT induces notable changes in the electronic and optical properties of the nanotube, including a substantial bandgap reduction of up to ≈40% for the specific orientation A. The

• for applications ranging from ultraviolet light-emitting diodes and injection lasers to flat-panel displays and sensors [15][16][17][18][19]. ZnS, a promising transition metal chalcogenide with a wide bandgap of approximately 3.7 eV, has shown remarkable potential in gas sensing applications

• Figure 4. The pristine ZnS NT exhibits a bandgap of 3.03 eV (Figure 3i), consistent with previous studies [38][40][54][55][56][57][58]. Upon CP adsorption, the bandgap values for orientations A, B, C, and D were reduced to 1.92, 2.27, 2.31, and 2.47 eV, respectively. This reduction in the bandgap is

Influence of ion beam current on the structural, optical, and mechanical properties of TiO₂ coatings: ion beam-assisted vs conventional electron beam evaporation

• Agata Obstarczyk and
• Urszula Wawrzaszek

Beilstein J. Nanotechnol. 2025, 16, 1097–1112, doi:10.3762/bjnano.16.81

• transmitted through the coating, but also information about the band structure of the materials from which it is made of. Therefore, to perform a comprehensive analysis of the band structure of the prepared titania films, optical bandgap energy (Egopt) and Urbach energy (Eu) were analyzed. The value of Egopt

• was calculated by extrapolating the linear portion of the curves [42] based on the plot of (αhν)1/2 as a function of photon energy (hν) (Figure 5). Based on the literature [43][44][45], titanium dioxide in the anatase phase is an indirect-bandgap semiconductor. In the case of the films deposited using

• the conventional EBE method and an additional Iibg of 3 A, the optical bandgap energy was equal to 3.23 eV, while increasing Iibg to 4 A led to a slight decrease of Egopt to 3.16 eV. After post-process annealing, the value of the optical bandgap energy decreased for the film deposited without any

Piezoelectricity of hexagonal boron nitrides improves bone tissue generation as tested on osteoblasts

• Sevin Adiguzel,
• Nilay Cicek,
• Zehra Cobandede,
• Feray B. Misirlioglu,
• Hulya Yilmaz and
• Mustafa Culha

Beilstein J. Nanotechnol. 2025, 16, 1068–1081, doi:10.3762/bjnano.16.78

• alternating boron and nitrogen atoms, with a bond length of ≈1.45 Å and an AA stacking arrangement held together by σ bonds. Between adjacent hBN layers, the B–N atoms are bound by weak van der Waals forces, contributing to a wide bandgap of 3.9–4.6 eV, influenced by significant electronegativity differences

• bandgap energy [46]. Supporting Information File 1, Figure S3 (f) illustrates that the hydrodynamic size of hBNs in aqueous suspension is approximately 120 nm. The zeta potential value is −42.5 ± 1.18 mV, indicating greater negativity than −30 mV, demonstrating stability [47]. Figure 1 shows PRFM

Soft materials nanoarchitectonics: liquid crystals, polymers, gels, biomaterials, and others

• Katsuhiko Ariga

Beilstein J. Nanotechnol. 2025, 16, 1025–1067, doi:10.3762/bjnano.16.77

• observed in the polarized smectic crystal phase with the chromophore tilted from the layer normal. It is conceivable that the bulk photovoltaic effect could result in the generation of an open circuit voltage that exceeds the bandgap of the active layer. Furthermore, the bulk photovoltaic effect in the

Heat-induced transformation of nickel-coated polycrystalline diamond film studied in situ by XPS and NEXAFS

• Olga V. Sedelnikova,
• Yuliya V. Fedoseeva,
• Dmitriy V. Gorodetskiy,
• Yuri N. Palyanov,
• Elena V. Shlyakhova,
• Eugene A. Maksimovskiy,
• Anna A. Makarova,
• Lyubov G. Bulusheva and
• Aleksandr V. Okotrub

Beilstein J. Nanotechnol. 2025, 16, 887–898, doi:10.3762/bjnano.16.67

• their distinct atomic structures. Diamond is a wide bandgap semiconductor, which makes it resistant to high voltages and ionizing radiation. In contrast, graphitic materials demonstrate excellent electrical conductivity. This divergence in physical properties has encouraged significant interest in

Ar⁺ implantation-induced tailoring of RF-sputtered ZnO films: structural, morphological, and optical properties

• Manu Bura,
• Divya Gupta,
• Arun Kumar and
• Sanjeev Aggarwal

Beilstein J. Nanotechnol. 2025, 16, 872–886, doi:10.3762/bjnano.16.66

• , Urbach energy, and optical bandgap. The low reflectance values of implanted films assure their suitability as transparent windows and anti-reflective coating in various optoelectronic devices. Keywords: AFM; diffuse reflectance; GXRD; polycrystalline; ZnO films; Introduction Zinc oxide has emerged as a

• promising material for device fabrication in different fields, namely, spintronics, nanoelectronics, and photonics [1][2]. It possesses a wide bandgap of 3.37 eV [3] and has a large exciton binding energy of about 60 meV [4], which assures the stability of ZnO film-based devices such as liquid crystal

• optical phonon modes is ascribed to the formation of oxygen vacancies, which are supposed to be electron carriers in ZnO. Therefore, the evolution of the A1 (LO) mode acts as indirect evidence of a rise in carrier concentration, which can in turn alter the optical bandgap. Moreover, the presence of

Insights into the electronic and atomic structures of cerium oxide-based ultrathin films and nanostructures using high-brilliance light sources

• Paola Luches and
• Federico Boscherini

Beilstein J. Nanotechnol. 2025, 16, 860–871, doi:10.3762/bjnano.16.65

• can be exploited to obtain insight into the processes following photoexcitation in photocatalysts, important for the rational optimization of these materials’ efficiency. In ceria, as well as in other semiconducting oxides, the formation of photoinduced small polarons after bandgap photoexcitation

• laser pulses at an energy lower than the cerium oxide bandgap. The four panels on the right of Figure 7 show the variation of the transient absorption as a function of the pump–probe delay time at selected FEL photon energies across the Ce N4,5 edge. The left panel of Figure 7 shows the steady-state Ce

• of this finding is that it provided a direct explanation for the observed sensitization of wide-bandgap oxides, such as cerium oxide, to the visible range through the coupling with suitable plasmonic metal NPs. The NPs convert the resonantly absorbed visible photons into excited charges in the oxide

Supramolecular hydration structure of graphene-based hydrogels: density functional theory, green chemistry and interface application

• Hon Nhien Le,
• Duy Khanh Nguyen,
• Minh Triet Dang,
• Huyen Trinh Nguyen,
• Thi Bang Tam Dao,
• Trung Do Nguyen,
• Chi Nhan Ha Thuc and
• Van Hieu Le

Beilstein J. Nanotechnol. 2025, 16, 806–822, doi:10.3762/bjnano.16.61

• , bandgap energy, and formation energy of the molecular system of bilayer graphene intercalated with a water layer. In the experimental aspect, green chemistry methods were applied for synthesizing GO nanosheets, rice-husk-derived silica gel (SG), nanosilica–zinc hydroxide nanoparticles (SG-ZH), and

• ). The intersheet distance is comparable to the values reported in other papers [29][30][31]. The bilayer graphene structure has a small bandgap of 0.06 eV which is slightly open in comparison to the zero bandgap of a single-layer graphene sheet. Besides, DFT modeling of the water-intercalated AB bilayer

• intersheet distance of 6.626 Å led to the intersheet binding energy of 0.04 eV/atom. A layer of water molecules in between two graphene sheets significantly declined the van der Waals force by 37.5% (from 0.064 to 0.040 eV). The bandgap of the water-intercalated bilayer graphene structure increased to 0.09

Morphology and properties of pyrite nanoparticles obtained by pulsed laser ablation in liquid and thin films for photodetection

• Akshana Parameswaran Sreekala,
• Bindu Krishnan,
• Rene Fabian Cienfuegos Pelaes,
• David Avellaneda Avellaneda,
• Josué Amílcar Aguilar-Martínez and
• Sadasivan Shaji

Beilstein J. Nanotechnol. 2025, 16, 785–805, doi:10.3762/bjnano.16.60

• a narrow bandgap (0.95 eV), high light absorption coefficient (≈105 cm−1), excellent properties in photoelectric conversion, and has enormous potential as an efficient photodetector system and in lithium batteries [1][2]. The prevalent forms of FeS2 are cubic-system pyrite and the orthorhombic

• -system marcasite crystal structure. Due to its low structural symmetry, marcasite FeS2 has a bandgap of only 0.34 eV and, as a result, it is not appropriate for use, particularly with solar energy absorption materials. Hence, the pyrite structure serves as a foundation for almost all studies of systems

• such properties [31]. For instance, it has been demonstrated that the wavelength-shifting characteristics of Si nanoparticles were caused by the effects of quantum-size confinement. The bandgap of silicon increased from its typical 1.1 eV in elemental form to nearly 3 eV in nanoparticle form, enhancing

Nanostructured materials characterized by scanning photoelectron spectromicroscopy

• Matteo Amati,
• Alexey S. Shkvarin,
• Alexander I. Merentsov,
• Alexander N. Titov,
• María Taeño,
• David Maestre,
• Sarah R. McKibbin,
• Zygmunt Milosz,
• Ana Cremades,
• Rainer Timm and
• Luca Gregoratti

Beilstein J. Nanotechnol. 2025, 16, 700–710, doi:10.3762/bjnano.16.54

• , optoelectronic, or photovoltaic devices, as they combine a direct bandgap of tunable size with high charge carrier mobility [20]. Furthermore, they can be grown on Si substrates [21][22], which enables integration with a well-established technology platform and constrains the use of high performance, but

• Ni3+ to reach charge neutrality. Additionally, NiO exhibits a wide bandgap, which also prompts considerable research interest. The properties of NiO are highly dependent on the synthesis method, owing to the variable dimensionality, morphology, crystalline orientation, and defect structure [44

Retrieval of B1 phase from high-pressure B2 phase for CdO nanoparticles by electronic excitations in Cd_xZn_1−xO composite thin films

• Arkaprava Das,
• Marcin Zając and
• Carla Bittencourt

Beilstein J. Nanotechnol. 2025, 16, 551–560, doi:10.3762/bjnano.16.43

• ; Introduction Zinc oxide (ZnO)-based thin films are of significant interest due to their wide bandgap value (3.37 eV at room temperature), transparent electrical conduction, and large excitonic binding energy (60 meV) [1]. In contrast, cadmium oxide (CdO) exhibits a lower bandgap of 2.2 eV, along with high

• electron mobility (>100 cm2/V/s) and high electrical conductivity (>1014 S/cm), demonstrating its potential for optoelectronic applications [2][3][4]. The incorporation of cadmium into ZnO effectively reduces the bandgap, rendering the thin films suitable for applications in the visible region of the

• electromagnetic spectrum [5]. Composite semiconducting thin films have garnered significant attention as their bandgap can be lowered without compromising mobility and conductivity. Beyond optoelectronic applications, CdO–ZnO-based alloys are also employed in gas-sensing technologies [6]. In prior investigations

Water in nanoporous hexagonal boron nitride nanosheets: a first-principles study

• Juliana A. Gonçalves,
• Ronaldo J. C. Batista and
• Marcia C. Barbosa

Beilstein J. Nanotechnol. 2025, 16, 510–519, doi:10.3762/bjnano.16.39

• , and high mechanical strength [20][21]. Additionally, it possesses unique properties compared to graphene, such as a wide bandgap, electrical insulation, and chemical inertness. Because of its remarkable mechanical properties and resistance to oxidation during the desalination process, h-BN can be used

Quantification of lead through rod-shaped silver-doped zinc oxide nanoparticles using an electrochemical approach

• Ravinder Lamba,
• Gaurav Bhanjana,
• Neeraj Dilbaghi,
• Vivek Gupta and
• Sandeep Kumar

Beilstein J. Nanotechnol. 2025, 16, 422–434, doi:10.3762/bjnano.16.33

• , India Department of Physics, Punjab Engineering College (Deemed to be University), Chandigarh, 160012, India 10.3762/bjnano.16.33 Abstract Special features of zinc oxide nanoparticles have drawn a lot of interest due to their wide bandgap, high surface area, photocatalytic activity, antimicrobial

• employed as effective electron mediators [9]. Zinc oxide nanoparticles have gained a lot of attention due to their unique features, such as wide bandgap (approximately 3.37 eV), excellent electron transportation, piezoelectric behavior, semiconductor nature, low toxicity, and enhanced electrochemical

• Ag@ZnO nanorods Figure 4a displays the optical spectra of Ag@ZnO NRs, which was obtained in the 200–600 nm wavelength range. The absorbance peak in this spectrum, which is moved toward a higher wavelength also known as redshift, is shown at 378 nm (for pure ZnO it is 362 nm) [20]. The bandgap energy

Tailoring of physical properties of RF-sputtered ZnTe films: role of substrate temperature

• Kafi Devi,
• Usha Rani,
• Arun Kumar,
• Divya Gupta and
• Sanjeev Aggarwal

Beilstein J. Nanotechnol. 2025, 16, 333–348, doi:10.3762/bjnano.16.25

• the optical bandgap of the films can be tuned from 1.47 ± 0.02 eV to 3.11 ± 0.14 eV. The surface morphology of the films studied using atomic force microscopy reveals that there is uniform grain growth on the surface. Various morphological parameters such as roughness, particle size, particle density

• , skewness, and kurtosis were determined. Current–voltage characteristics indicate that the conductivity of the films increased with substrate temperature. The observed variations in structural, morphological, and optical parameters have been discussed and correlated. The wide bandgap (3.11 eV), high

• crystallinity, high transmittance, and high conductivity of the ZnTe film produced at 600 °C make it a suitable candidate for use as a buffer layer in solar cell applications. Keywords: bandgap; physical properties; RF sputtering; substrate temperature; ZnTe; Introduction The industrialization and burning of

Emerging strategies in the sustainable removal of antibiotics using semiconductor-based photocatalysts

• Yunus Ahmed,
• Keya Rani Dutta,
• Parul Akhtar,
• Md. Arif Hossen,
• Md. Jahangir Alam,
• Obaid A. Alharbi,
• Hamad AlMohamadi and
• Abdul Wahab Mohammad

Beilstein J. Nanotechnol. 2025, 16, 264–285, doi:10.3762/bjnano.16.21

• bandgap, electrons (e−) in the valence band (VB) transition to the conduction band (CB), resulting in the formation of holes (h+) in the VB (photocatalyst + hν → photocatalyst + h+ + e−) [54][55]. Afterwards, the electrons and holes are effectively separated and move toward the surface of the

• (antibiotics + HO• and/or O2•− → CO2 + H2O). Mechanisms of metal, nonmetal, or co-doped photocatalysts The large bandgap and high electron–hole recombination rate of traditional and single semiconductor photocatalysts limit their effectiveness under visible light, which hinders their practical application. To

• CB of the material [58][59]. This action serves to reduce the bandgap, which in turn extends the absorption wavelength edge towards the region of visible light [60][61]. The idea of modifying semiconductor materials in the second generation involves the process of co-doping with both metal and