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Catalogue · open problems

Open problems

Every material page ends with the questions about it that are still open. Here are all 250, from 74 materials, sorted by theme. Each links to the page that explains its background.

Topology

Edge, surface and hinge states, Majorana modes and quantised Hall effects – and whether they survive in thin flakes and at higher temperatures.

  • Can zero-field fractional Chern insulator states be stabilised at higher temperatures and used for topologically protected qubits? Twisted graphene
  • What decides the competition between fractional Chern insulators, charge order and superconductivity in twisted MoTe2? MoTe₂
  • Does the topological character of Td-MoTe2 survive in few-layer samples? MoTe₂
  • Is the monolayer bulk gap a single-particle topological gap, or an excitonic-insulator gap stabilised by interactions? WTe₂
  • How do the quantum spin Hall edges couple to gate-induced superconductivity – and can they host Majorana modes? WTe₂
  • Can NbSe2-based junctions host topological superconductivity when combined with magnets or topological insulators? NbSe₂, NbS₂, NbTe₂
  • Do the ditellurides keep their type-II Dirac bands in few-layer flakes, and can PdTe2’s superconductivity be made topological? PtSe₂, PdSe₂, ditellurides
  • Are the zero-energy vortex states in FeTe1−xSex Majorana modes, and why do only some vortices show them? FeSe, FeTeSe
  • Can bulk carriers be eliminated so that topological surface transport dominates at room temperature? Bi₂Se₃, Bi₂Te₃
  • Can the quantum anomalous Hall effect be pushed well above a few kelvin for metrology or dissipationless interconnects? Bi₂Se₃, Bi₂Te₃
  • Is ZrTe5 a strong or a weak topological insulator, or a Dirac semimetal – and how much do strain, tellurium vacancies and temperature decide the answer in a given crystal? TaIrTe₄, ZrTe₅, HfTe₅
  • What is the correlated insulating state in monolayer TaIrTe4, and could it host fractional topological phases? TaIrTe₄, ZrTe₅, HfTe₅
  • Does the superconducting state inherit anything topological from the nodal lines, or is it conventional pairing in a topological metal? PbTaSe₂
  • Are the zero-bias features reported in vortex cores Majorana modes, or ordinary bound states? PbTaSe₂
  • How do the nodal lines evolve as crystals are thinned to a few layers? PbTaSe₂
  • Are there Majorana flat bands at surface step edges, as the nodal i-wave gap implies, and can they be detected? PtBi₂
  • How thin can PtBi2 become before its Weyl nodes, Fermi arcs and surface superconductivity change – and can a monolayer be isolated at all? PtBi₂
  • Can the surface Dirac states be gapped in a controlled way by the magnetic order, as the axion picture requires? EuSn₂As₂
  • Can a heavy Xene be grown on a substrate weak enough to preserve its predicted topological gap – and can helical edge conduction be shown in transport? Germanene, stanene
  • Can functionalised versions such as fluorinated stanene deliver quantum spin Hall behaviour at room temperature? Germanene, stanene
  • Are there topological magnons, and what opens the gap at the magnon Dirac points? CrI₃, VI₃, TiCl₃
  • Is the field-induced phase a quantum spin liquid, and is its thermal Hall effect carried by Majorana modes or by magnons? α-RuCl₃
  • Is Nb3Br8 an obstructed atomic insulator, and is that what makes its Josephson junctions non-reciprocal? Nb₃Cl₈, Nb₃Br₈, Nb₃I₈
  • Why is the quantum anomalous Hall effect so hard to reproduce between flakes, and can it survive to higher temperatures? MnBi₂Te₄
  • Can defect control or MnBi4Te7-type intergrowths give a robust ferromagnetic topological insulator? MnBi₂Te₄
  • Is BiTeCl a topological insulator at ambient pressure, or are its surface states topologically trivial? BiTeX
  • Can hinge conduction be measured in transport, cleanly separated from bulk and surface contributions? Bi₄Br₄
  • How robust are the hinge states to disorder, steps and the stacking faults that these chain crystals form easily? Bi₄Br₄
  • Can the topological phase be switched on purpose by strain or by changing the stacking? Bi₄Br₄

Superconductivity and charge order

Pairing, charge-density waves, Mott and Kondo physics: states in which the electrons act together.

  • What is the pairing mechanism – phonon-mediated, or driven by electronic correlations and the quantum geometry of the flat bands? Twisted graphene
  • Are predicted magnetism and Ising superconductivity in the family real, or artefacts of idealised calculations? MoSi₂N₄
  • Can moiré WSe2 heterostructures realise tunable Hubbard-model physics at practical temperatures? WSe₂
  • How do Ising superconductivity and the charge density wave compete or cooperate in the monolayer limit? NbSe₂, NbS₂, NbTe₂
  • Is pairing in few-layer NbSe2 purely conventional, or are triplet or nematic components present? NbSe₂, NbS₂, NbTe₂
  • Monolayer NbS2 has been reported to show a charge density wave that bulk crystals lack – what switches it on: reduced dimensionality, the substrate, or strain? NbSe₂, NbS₂, NbTe₂
  • Is bulk 1T-TaS2 a Mott insulator, or a band insulator created by pairing of layers along the stacking direction? TaS₂, TaSe₂
  • Is the charge-density wave in TiSe2 driven by exciton condensation, by electron–phonon coupling, or by both working together? TiSe₂, ZrSe₂, HfSe₂
  • Why does a charge-density wave appear in single-layer TiTe2 but not in thicker flakes? TiSe₂, ZrSe₂, HfSe₂
  • What lifts superconductivity in single-layer FeSe on SrTiO3: electron doping, coupling to substrate phonons, or both? FeSe, FeTeSe
  • How does nematic order form in FeSe without magnetism, and does it help or hinder pairing? FeSe, FeTeSe
  • Do the bulk Hall plateaus in ZrTe5 and HfTe5 need a field-induced charge-density wave, or do they follow from a low-density Dirac band in the quantum limit? TaIrTe₄, ZrTe₅, HfTe₅
  • How much of the charge order is nesting and how much is momentum-dependent electron–phonon coupling – the same argument as in the dichalcogenides, but with a cleaner Fermi surface LaTe₃, GdTe₃, ErTe₃
  • What decides where the second, perpendicular modulation appears in the series, and how do the two orders coexist? LaTe₃, GdTe₃, ErTe₃
  • How do the charge-density waves of NbSe3 and ZrTe3 change when crystals are thinned to a few chains or a single layer, and does sliding survive? TiS₃, ZrTe₃, NbSe₃, TaSe₃
  • Does TiS3 form a charge-density wave or undergo a metal–insulator transition at low temperature? The same transistor data have been read both ways TiS₃, ZrTe₃, NbSe₃, TaSe₃
  • Is the 326 K transition driven by electron–hole pairing, by a lattice instability, or by both together – and how much of the gap does each contribute? Ta₂NiSe₅
  • Which experiment can separate an excitonic and a structural order parameter that share the same symmetry? Ta₂NiSe₅
  • Does any part of the ordered phase behave like an exciton condensate able to carry energy with little dissipation? Ta₂NiSe₅
  • Why do the surfaces superconduct near 10 K when the bulk barely does – and why does the measured surface gap vary so much from place to place? PtBi₂
  • Can the two superconducting surfaces of a flake be used as a built-in Josephson junction? PtBi₂
  • How does Kondo screening change as the crystal is thinned, and is there a true monolayer limit for heavy-fermion behaviour? CeSiI
  • What is the interplay between frustrated magnetism and Kondo screening here? CeSiI
  • Can gating or strain drive the material across a quantum critical point, the way pressure does in bulk heavy fermions? CeSiI
  • Do heterostructures with other van der Waals layers change the Kondo temperature in a controlled way? CeSiI
  • Does the charge order really break time-reversal symmetry? Muon and optical experiments disagree, and the answer decides whether this is a loop-current state CsV₃Sb₅, KV₃Sb₅
  • Is the superconductivity conventional? Reports range from a nodeless multigap state to a pair density wave with strong coupling CsV₃Sb₅, KV₃Sb₅
  • What sets the 2×2 (and 4a0) patterns: van Hove singularities at the Fermi level, phonons, or both? CsV₃Sb₅, KV₃Sb₅
  • Does the predicted superconductivity appear once substrate charge transfer is controlled? Borophene
  • What drives the 90 K transition of Nb3Cl8 – charge disproportionation between layers, dimerisation of layers, or singlet formation – and does it survive in a monolayer? Nb₃Cl₈, Nb₃Br₈, Nb₃I₈
  • What is the microscopic nature of the spin-correlated exciton in NiPS3? MPS₃
  • What happens at the graphene/CrOCl interface: charge transfer into CrOCl states, correlations within graphene, or both? NbOI₂, CrOCl, TiOCl
  • Why does the transition temperature rise as electrons are removed – are magnetic or valley fluctuations doing the pairing? ZrNCl, HfNCl
  • Is the pairing state the same in the intercalated bulk and in the ion-gated surface layer? ZrNCl, HfNCl
  • What is the nature of the metallic ground state seen in a magnetic field, and is it a genuine bosonic metal? ZrNCl, HfNCl
  • How are superconductivity and magnetism, normally incompatible, arranged at the same interface? LaAlO₃/SrTiO₃

Magnetism

What keeps magnetic order alive in a few layers, how to raise its temperature, and what it couples to.

  • Is there a quantum spin liquid in 1T-TaS2 or in related 1T-TaSe2 monolayers? TaS₂, TaSe₂
  • Where did the room-temperature magnetic signal of the 2018 VSe2 monolayers come from – defects, the substrate, or a state that is hard to reproduce? VSe₂, CrTe₂, CrSe₂
  • Why does monolayer CrTe2 turn antiferromagnetic while the bulk is ferromagnetic, and how much do the substrate and strain decide it? VSe₂, CrTe₂, CrSe₂
  • Can chromium self-intercalation, which turns CrTe2 into Cr1+δTe2, be controlled well enough to set the magnetism layer by layer? VSe₂, CrTe₂, CrSe₂
  • Does hole-doped monolayer InSe become magnetic, as its ‘Mexican hat’ valence band suggests? InSe, GaSe
  • What happens in thin flakes, where the number of europium layers decides the net magnetisation? EuSn₂As₂
  • How does the ordering temperature limit what can be measured, and can it be raised by substitution? EuSn₂As₂
  • How do chirality and spin–orbit coupling combine to produce the reported current-induced spin polarisation? Tellurene
  • Why does stacking in thin CrI3 differ from bulk, and can stacking-controlled magnetism be engineered deliberately? CrI₃, VI₃, TiCl₃
  • Can the Curie temperature be raised towards room temperature by strain, doping or pressure? CrI₃, VI₃, TiCl₃
  • Which interactions beyond Kitaev exchange – Heisenberg, off-diagonal Γ, further-neighbour terms – set the ground state, and how large are they? α-RuCl₃
  • What happens to the magnetism in a monolayer, where stacking disappears and a substrate can dope the sheet? α-RuCl₃
  • How do frustration, Kitaev-type exchange and interlayer coupling combine to set the spiral in NiI2? NiI₂, FeCl₂, CoCl₂
  • Can layered antiferromagnets such as FeCl2, whose layers a field can realign, serve as switchable magnetic layers in van der Waals heterostructures? NiI₂, FeCl₂, CoCl₂
  • Do monolayers host a quantum spin liquid on the frustrated lattice of trimers? Nb₃Cl₈, Nb₃Br₈, Nb₃I₈
  • Can doping or interface engineering raise its Curie temperature while keeping it semiconducting? Cr₂Ge₂Te₆
  • How exactly does its weak anisotropy sustain order in the thinnest layers at finite temperature? Cr₂Ge₂Te₆
  • Can strain or pressure flip the anisotropy from easy-axis to easy-plane before the ferromagnetism itself is lost? Cr₂Ge₂Te₆
  • What sets the quasi-one-dimensional character of its excitons and their coupling to magnetic order? CrSBr
  • Can magnon–exciton coupling be used for coherent conversion between microwave and optical signals? CrSBr
  • Can its ordering temperature be pushed towards room temperature? CrSBr
  • Fe3GaTe2 orders above room temperature without gating – what lifts its Curie temperature so far above Fe3GeTe2’s, and can thin layers be made air-stable? Fe₃GeTe₂, Fe₃GaTe₂
  • What stabilises skyrmions and other chiral textures in a nominally centrosymmetric crystal – interfaces, defects or dipolar effects? Fe₃GeTe₂, Fe₃GaTe₂
  • Can wafer-scale MBE films match the magnetic quality of exfoliated flakes? Fe₃GeTe₂, Fe₃GaTe₂
  • How exactly does XY-type order in NiPS3 vanish at the monolayer – is there a Berezinskii–Kosterlitz–Thouless-like regime? MPS₃
  • Can antiferromagnetic order in insulating layers be read and written electrically? MPS₃
  • How do the ordering temperature and the spin-flop field change layer by layer, and what sets the monolayer’s Curie temperature? CrPS₄
  • Which exchange paths – along the chains, between chains, between layers – decide the ground state, and how much do long-range interactions matter? CrPS₄
  • Can magnons be injected, switched and read out in an all-van-der-Waals circuit, without deposited platinum contacts? CrPS₄
  • What are the localised excitons in its optical spectra, and how do they couple to the magnetic order? CrPS₄
  • Is there a magnetic gap in the surface Dirac cone, and why do different measurements disagree? MnBi₂Te₄

Ferroelectric and polar order

Switchable polarisation and built-in fields, and what thinning does to them.

  • What is the mechanism of switchable polarisation in few-layer WTe2, given that it conducts? WTe₂
  • Can the built-in dipole be measured directly and quantitatively, rather than inferred from DFT? Janus MoSSe
  • Do predicted Rashba splittings survive substrate interactions and disorder in real samples? Janus MoSSe
  • SnS stays ferroelectric up to about 15 layers whether the layer count is odd or even – what stacking makes that possible, and can it be controlled? SnSe, GeSe
  • Can growth deliver uniform thin films with controlled polarisation domains, given how hard exfoliation is? SnSe, GeSe
  • How thin can CuInP2S6 be and stay ferroelectric? Early probe measurements lost polarisation below ~50 nm, later devices switched at ~4 nm – how much do electrodes, screening and mobile copper decide? CuInP₂S₆, CuCrP₂S₆
  • In a given switching loop, how much of the signal is ferroelectric polarisation and how much is copper-ion motion? CuInP₂S₆, CuCrP₂S₆
  • Can the high-polarisation state, with copper in the van der Waals gap, be stabilised and used at room temperature? CuInP₂S₆, CuCrP₂S₆
  • How does a ferroelectric that is also a semiconductor keep its polarisation when mobile carriers can screen it or leak – and what limits retention in transistors? In₂Se₃
  • Can wafer-scale films be grown at back-end-of-line temperatures without losing ferroelectric switching? In₂Se₃
  • Can the competing ferroelectric and antiferroelectric orders of β′-In2Se3 be switched on purpose? In₂Se₃
  • Does a single layer of NiI2 really keep its multiferroic order, and which measurements can prove it unambiguously? NiI₂, FeCl₂, CoCl₂
  • How large is the built-in electric field across one layer, and can it drive spin splitting or photocatalysis in devices? RhSeCl, RhTeCl
  • Its polar symmetry makes RhSeCl piezoelectric and pyroelectric in principle – how large are these responses? RhSeCl, RhTeCl
  • How does the Rashba splitting change in a single layer, where neighbouring layers no longer screen the built-in field? BiTeX
  • Can the polar stacking be used for a bulk photovoltaic effect or switchable polarisation in thin flakes? BiTeX
  • Can the ferroelectric polarisation of NbOX2 be switched electrically in thin flakes, and how does switching change the nonlinear optical response? NbOI₂, CrOCl, TiOCl
  • Can SbSI’s Curie point be pushed comfortably above room temperature by substitution or strain, without losing the response? SbSI, BiSI
  • What do isolated chains do – can a single chain or a few-chain ribbon be measured, and does the polarisation survive? SbSI, BiSI
  • How much of the reported piezoelectric response belongs to the material and how much to the composite it is measured in? SbSI, BiSI
  • What mix of electronic reconstruction at the polar interface, oxygen vacancies and cation intermixing creates the electron gas? LaAlO₃/SrTiO₃

Band structure

Band gaps and Dirac cones, and whether a layer on its substrate is what the calculations describe.

  • Is there a way to open a technologically useful band gap without sacrificing the carrier mobility that made graphene attractive? Graphene
  • Do the predicted Dirac cones of α-, β- and 6,6,12-graphyne survive in real, substrate-supported samples? Graphyne
  • Is the monolayer gap direct or indirect? Reports conflict. ReS₂, ReSe₂
  • Where exactly does the semiconductor–semimetal transition occur, and how do defects and interlayer coupling shift it? PtSe₂, PdSe₂, ditellurides
  • Is the structure on Ag(111) genuinely silicene or a Si–Ag surface alloy, and does any Dirac character survive on any substrate? Silicene
  • Can a weakly interacting, insulating substrate support silicene growth so its intrinsic electronic structure can be measured? Silicene
  • How much of the measured electronic structure reflects surface alloying rather than a genuine Xene? Germanene, stanene
  • Is the predicted wide gap of monolayer antimonene confirmed, given that exfoliated flakes are rarely single layers? Antimonene, bismuthene
  • Do thin, exfoliated or grown SnO layers keep the bulk valence-band structure? SnO

Energy, chemistry and catalysis

Catalysts, solar cells, membranes and batteries, and the chemistry of the layers themselves.

  • What is the atomic structure of GO, and can batch-to-batch variability be reduced enough for regulated uses such as medicine? GO / rGO
  • Can swelling of GO laminates in water be suppressed at membrane scale without sacrificing water flux? GO / rGO
  • Which reduction chemistry restores the most conductivity without toxic reagents or high temperatures? GO / rGO
  • How much of the reported catalytic activity comes from the carbon network rather than from residual copper? Graphyne
  • Can quantum efficiency be raised enough, without platinum co-catalysts, for practical solar hydrogen? g-C₃N₄
  • Are reported performance gains comparable between labs, given differences in light sources and measurement protocols? g-C₃N₄
  • Does the record thermoelectric performance of SnSe survive in polycrystalline or thin-film forms? SnSe, GeSe
  • What limits oxidation stability, and can water-based inks reach shelf lives of years? MXenes
  • Are the poor BiSI solar efficiencies really a contact-alignment problem, and do the suggested device stacks fix them? SbSI, BiSI
  • Are reported ammonia yields from photocatalytic nitrogen fixation real, given how easily trace ammonia and nitrogen-containing impurities contaminate such measurements? BiOCl, BiOBr, BiOI
  • Why do BiOI solar cells still convert far less light than their absorption allows, and can predicted defect tolerance be turned into efficiency? BiOCl, BiOBr, BiOI
  • What is the true active site in NiFe-LDH oxygen evolution – iron centres, Ni–Fe pairs, or oxyhydroxide surfaces that form during operation? LDHs
  • Can activity be sustained at industrial current densities for thousands of hours, given iron segregation and dissolution? LDHs
  • Can a lead-free heavy halide match PbI2’s stopping power without its toxicity? PbI₂, BiI₃
  • How exactly do 2D layers stabilise 3D perovskite cells – chemical passivation, ion blocking or both – and do they degrade themselves? 2D perovskites
  • Can conductive MOFs survive the chemistry of real electrochemical devices for long periods? 2D MOFs

Light and excitons

Excitons, single-photon emitters, nonlinear optics and photodetectors.

  • What are the atomic structures of the room-temperature single-photon emitters, and can they be placed deterministically? hBN
  • Why does photoluminescence efficiency vary so widely between CVD monolayers, and which defects cause the non-radiative losses? WS₂
  • Can interlayer excitons in WS2-based heterobilayers be used for room-temperature excitonic devices? WS₂
  • How do excitons dress into Fermi polarons as carriers are added, and what does this reveal about many-body interactions in two dimensions? MoSe₂
  • Can MoSe2-based moiré heterostructures host controllable arrays of quantum emitters? MoSe₂
  • What limits exciton coherence in the cleanest available samples? MoSe₂
  • What is the microscopic origin of WSe2 quantum emitters – strain-localised dark excitons, defects, or both? WSe₂
  • Which polytype forms under which growth conditions, and how does it affect optical properties? SnS₂, SnSe₂
  • Do light-induced states such as the transient perpendicular wave in LaTe3 have equilibrium counterparts, or are they genuinely new phases? LaTe₃, GdTe₃, ErTe₃
  • Which member of the family is the best infrared detector – the comparison has not been made under common conditions GeAs, GeP, SiAs
  • How do the anisotropic excitons evolve with layer number and dielectric environment? Phosphorene
  • Does a single Janus layer keep the strong second-harmonic response, and how does the response depend on the coupling between layers? RhSeCl, RhTeCl
  • What makes plasmons in MoOCl2 low-loss even though its electron transport looks like that of a bad metal, and can the effect be found in other crystals? NbOI₂, CrOCl, TiOCl
  • How do oxygen vacancies and cation disorder govern the electronic and optical behaviour of atomically thin oxides? Oxide nanosheets
  • Does monolayer PbI2 behave like the bulk optically, and can it be grown rather than exfoliated into flakes large enough to measure? PbI₂, BiI₃
  • Can films be grown with the uniformity and area that light-emitting devices need, rather than as crystals at an interface? Mithrene (AgSePh)
  • What limits the emission linewidth and quantum yield – ligand disorder, silver vacancies, or exciton–phonon coupling? Mithrene (AgSePh)
  • How far can the chemistry be pushed: which ligand and metal combinations give stable layers with a designed emission colour? Mithrene (AgSePh)

Devices and electronics

Contacts, doping, mobility, dielectrics and cost: what stands between a good flake and a working chip.

  • Once grain boundaries are eliminated, what limits room-temperature mobility in large-area films: wrinkles, strain fluctuations or residual charged impurities? Graphene
  • Can wafer-scale single-crystal hBN match the low defect density of the best bulk crystals – and be integrated as a dielectric in CMOS processes? hBN
  • Can monolayer hBN work as a reliable gate dielectric despite direct tunnelling leakage? hBN
  • What is the true structure of typical ‘g-C3N4’ samples, and how much does incomplete condensation limit charge transport? g-C₃N₄
  • Can Ca2N be thinned to a single layer, and does the interlayer electron gas survive when there is only one gap? Ca₂N
  • How can flakes be passivated well enough to make devices without losing the electride character? Ca₂N
  • What is the intrinsic mobility of the interlayer electrons, separated from defects introduced during exfoliation? Ca₂N
  • What limits carrier mobility and contact resistance in grown films, where measurements so far lag behind the calculations? MoSi₂N₄
  • What limits mobility in wafer-scale films – chalcogen vacancies, grain boundaries, interface traps or remote phonons – and which of these can be engineered away? MoS₂
  • Can a stable p-type MoS2, or a well-matched p-type partner, be achieved for complementary 2D logic? MoS₂
  • Can WS2 transistors reach the device-to-device uniformity needed for integration at scale? WS₂
  • Can p-type contact resistance to WSe2 be brought down to the level already achieved for n-type MoS2? WSe₂
  • Can 2H/1T′ phase patterning be made reproducible and stable enough for a lateral-contact technology? MoTe₂
  • What is the hidden metallic state, and can it be written and erased reliably in devices at practical temperatures? TaS₂, TaSe₂
  • Can native HfO2 or ZrO2 grown on HfSe2 and ZrSe2 approach the interface quality of silicon with its oxide? TiSe₂, ZrSe₂, HfSe₂
  • Does rhenium’s scarcity rule out anything beyond niche optoelectronics? ReS₂, ReSe₂
  • Can low-temperature PtSe2 films become crystalline enough to rival exfoliated flakes in mobility? PtSe₂, PdSe₂, ditellurides
  • Is the cost of platinum acceptable for any high-volume application? PtSe₂, PdSe₂, ditellurides
  • Can InSe be protected from degradation at wafer scale without losing its mobility? InSe, GaSe
  • Can the ballistic transport of exfoliated flakes be reproduced in uniformly grown films? InSe, GaSe
  • Can the high native n-doping of SnSe2 be controlled? SnS₂, SnSe₂
  • Can SnSe2-based tunnel transistors achieve steep switching reproducibly? SnS₂, SnSe₂
  • How robust is spin–orbit torque efficiency in industrially sputtered topological insulator films? Bi₂Se₃, Bi₂Te₃
  • Can room-temperature nonlinear Hall rectifiers and spin–orbit torque layers be made from grown films rather than exfoliated flakes? TaIrTe₄, ZrTe₅, HfTe₅
  • What limits the open-circuit voltage – bulk defects, band tails, or the interfaces with the contact layers? Sb₂Se₃, Sb₂S₃
  • Can doping be controlled deliberately, rather than through selenium stoichiometry? Sb₂Se₃, Sb₂S₃
  • Do single ribbons or thin flakes behave like the bulk, and is there a device case for isolating them? Sb₂Se₃, Sb₂S₃
  • Can the high mobility of GdTe3 survive thinning to a monolayer and the transfer steps a device needs? LaTe₃, GdTe₃, ErTe₃
  • Can the transition be raised far enough above room temperature for memory that must survive chip operating temperatures? CuInP₂S₆, CuCrP₂S₆
  • Can measured TiS3 mobilities approach the ~10,000 cm2 V−1 s−1 predicted along the chains of a monolayer, when today’s values are more than a hundred times lower? TiS₃, ZrTe₃, NbSe₃, TaSe₃
  • Can quasi-1D metal wires be grown in place, at back-end-of-line temperatures, and tested as interconnects below the size where copper fails? TiS₃, ZrTe₃, NbSe₃, TaSe₃
  • Which polymorph and stacking sits in a given device, and how do α, β and β′ phases convert during growth, processing and switching? In₂Se₃
  • How large is the intrinsic mobility, and how much of the measured transport is hopping between defects rather than band conduction? GeAs, GeP, SiAs
  • Can crystals be grown without the high-pressure step, in sizes and quality that support device work? GeAs, GeP, SiAs
  • Can encapsulation make silicene devices last long enough to be useful? Silicene
  • Can scalable, air-stable passivation be achieved without degrading mobility? Phosphorene
  • Can quantised edge conduction in bismuthene on SiC be demonstrated in transport, not only in spectroscopy? Antimonene, bismuthene
  • Can the mobility of solution-grown flakes be reproduced in uniform, thickness-controlled wafer-scale films? Tellurene
  • Is tellurene a practical p-type partner for n-type TMDCs in complementary 2D logic? Tellurene
  • Are chiral orbital currents real, and can they be imaged rather than inferred from transport? Mn₃Si₂Te₆
  • Why does the resistance depend so strongly on field direction rather than magnitude? Mn₃Si₂Te₆
  • Can the doped layer be isolated as a free-standing 2D crystal rather than being made at a surface or between layers? ZrNCl, HfNCl
  • Can contacts and doping be controlled well enough to turn high Hall mobility into short-channel transistor performance? Bi₂O₂Se
  • How reliable is the native Bi2SeO5 dielectric – leakage, breakdown, interface traps – compared with deposited high-κ oxides? Bi₂O₂Se
  • Can hole mobility be pushed from a few cm2 V−1 s−1 to something competitive with n-type oxides like IGZO? SnO
  • What limits hole doping, and can acceptors be introduced without wrecking the lattice? SnO
  • Can oxide nanosheet dielectrics deliver reliable leakage and breakdown statistics when integrated with 2D semiconductors? Oxide nanosheets
  • What sets the leakage current in thick detector films – bulk defects, electrodes, or grain boundaries? PbI₂, BiI₃
  • Is conduction in π-stacked MOFs band-like or hopping, and how much is limited by grain boundaries rather than intrinsic properties? 2D MOFs
  • Can large, single-crystalline, few-layer films be grown to measure intrinsic transport? 2D MOFs
  • Can large-area single-crystalline COF films be grown, given that polycrystallinity and stacking faults limit transport and separation? 2D COFs

Making and understanding the crystals

Growth at scale, stability, defects, and what the structure of a layer really is.

  • Can single-crystal graphene be transferred at 300 mm scale without cracks, wrinkles and polymer residue – or grown directly on the target substrate at CMOS-compatible temperatures? Graphene
  • How much of the measured phase diagram is intrinsic, and how much is set by twist-angle disorder, heterostrain and hBN alignment? Twisted graphene
  • Can highly crystalline, single-layer graphdiyne be made over large areas, so that measured properties can finally be compared with theory? Graphyne
  • Which other layered electrides can be exfoliated the same way? Ca₂N
  • Can any of the predicted MA2Z4 compounds beyond MoSi2N4 and WSi2N4 be grown? MoSi₂N₄
  • Does growth ever give a controlled number of layers, given there is no bulk crystal to exfoliate from? MoSi₂N₄
  • How many reported ‘sulfur vacancies’ are really oxygen substitutions or antisite defects? MoS₂
  • Why is interlayer coupling so weak, and is it really as weak as early experiments suggested? ReS₂, ReSe₂
  • How complete is the conversion – what fraction of a ‘Janus’ flake is truly MXY, and how are alloyed regions and vacancies distributed? Janus MoSSe
  • Can Janus layers reach the crystalline quality of exfoliated parent monolayers, and at wafer scale? Janus MoSSe
  • How far can ribbon alignment be controlled across a full film, and what happens at the boundaries that are not aligned? Sb₂Se₃, Sb₂S₃
  • The ordered phase is already confined to single layers in bulk – what happens to it in an isolated monolayer? Ta₂NiSe₅
  • Do monolayers behave as the calculations predict, and can they be isolated reliably? GeAs, GeP, SiAs
  • How far can the thin-flake route go – can a monolayer be isolated, and does either order survive it? CsV₃Sb₅, KV₃Sb₅
  • Which polymorph is preferred on weakly interacting substrates, and can a free-standing phase be stabilised? Borophene
  • Can borophene be transferred intact from its growth metal? Borophene
  • Can crystalline black phosphorus films be grown directly at wafer scale? Phosphorene
  • How do these layers behave electronically once removed from their growth substrates? Antimonene, bismuthene
  • Can terminations be controlled uniformly, instead of the mixed –O/–OH/–F surfaces most etches produce? MXenes
  • Can fluorine-free, scalable synthesis match the flake quality of HF and LiF–HCl routes? MXenes
  • Which of the many predicted compositions are synthetically accessible, and are any of them semiconductors in practice? MXenes
  • Does the Ti–Ti pairing of bulk TiCl3 survive in thin flakes, and what does a single layer do instead? CrI₃, VI₃, TiCl₃
  • What ordering in mixed-halide CrX3 crystals produces the extra X-ray reflections that neither parent structure explains? CrI₃, VI₃, TiCl₃
  • Does the effect survive in thin flakes, where surfaces and strain matter more? Mn₃Si₂Te₆
  • How is charge shared between selenium and chlorine? Structural and magnetic studies describe Rh(III), while X-ray absorption and photoelectron spectra have been read as both anions carrying the same charge RhSeCl, RhTeCl
  • How much of the photoactivity comes from the internal electric field, from exposed facets and from vacancies, when a typical synthesis changes all three at once? BiOCl, BiOBr, BiOI
  • Can clean exfoliated or vapour-grown flakes separate the intrinsic properties of BiOX from those of defect-rich solution nanosheets? BiOCl, BiOBr, BiOI
  • Without a van der Waals gap, can Bi2O2Se be thinned reliably to a chosen number of layers, or does everything depend on growth? Bi₂O₂Se
  • What sets the electron density in as-grown crystals, and can selenium vacancies be tuned or compensated? Bi₂O₂Se
  • How can films be made phase-pure and stable when the compound itself is metastable? SnO
  • Can deterministic, wafer-scale assembly of nanosheets replace random restacking? Oxide nanosheets
  • Can freestanding oxide membranes combine oxide interface physics with van der Waals stacking? LaAlO₃/SrTiO₃
  • Do delaminated single layers restack inside electrodes and lose their advantage? LDHs
  • Can crystal quality reach the level where BiI3 and PbI2 detectors deliver spectroscopic resolution, rather than only counting events? PbI₂, BiI₃
  • Can phase-pure films with a single n be deposited uniformly, instead of the mixed-n distributions typical of solution processing? 2D perovskites
  • Can lead-free (for example tin-based) layered perovskites reach useful stability? 2D perovskites
  • Does the inorganic layer behave as a genuine two-dimensional semiconductor when isolated to a single sheet? Mithrene (AgSePh)
  • Reversible bonds give crystals but poor chemical stability – can irreversible linkages still yield long-range order? 2D COFs
  • Are 2D polymers such as 2DPA-1 truly two-dimensional throughout, and how do they compare with graphene as barrier materials at scale? 2D COFs