Copper-Oxide Photo-Memristor
A light-programmable memristor from flame-oxidized copper — the cheapest real optical synapse you can build (~$5, a torch, and a multimeter). For ALMAWARE / Eran neuromorphic R&D. Built for Iddo, 2026-06-24.
1 · Why copper oxide is the perfect cheap synapse
Heat a piece of copper in air and you grow two oxides at once. That single layer does both things a synapse needs:
- It remembers (memristor). Copper is a mobile ion. Under a small voltage, Cu⁺ ions drift through the oxide and grow/dissolve a conductive filament → the device switches between a Low-Resistance State (LRS) and High-Resistance State (HRS) and holds it. That's an ECM (electrochemical-metallization) memristor — the same class as your silver/Ag₂S work, but copper is cheaper and easier.
- It sees (photo). Cu₂O is a real photo-semiconductor (bandgap ≈ 2.0 eV) — light frees carriers and its conductance rises. This is the 1920s "copper-oxide photocell," rediscovered.
Put them together and you get a photo-memristor: a weight that can be set by voltage AND nudged by light — a light-programmable optical synapse, from a penny.
copper (bottom electrode + ion source)
│
┌────▼─────────────────────┐
│ CuO (black, ~1.4 eV) │ ← flaky outer skin (often scrubbed off)
│ Cu₂O (red, ~2.0 eV) │ ← the GOOD layer: photoactive + switches
└────▲─────────────────────┘
│
top contact (Ag paint / pressed probe / Al) ── light shines in here ──▶ ☀
2 · The two oxides (know your material)
| Oxide | Color | Bandgap | Role |
|---|
| Cu₂O (cuprous) | red / orange | ~2.0 eV (absorbs blue–green) | the photoactive, switching layer — the one you want |
| CuO (cupric) | black | ~1.2–1.5 eV (absorbs red–near-IR) | flaky outer skin; useful later for red/IR channel |
A flame grows a black CuO skin over a red Cu₂O layer. The classic trick: heat, then let it cool — the differential contraction makes the brittle black CuO flake off, exposing the clean red Cu₂O underneath.
3 · The build — honest tier ladder
TIER 0 Prove the photo-response — flame-oxidize + wet cell (~$5, today)
Parts
- Clean copper — sheet, flashing, a wide pipe flattened, or a pre-1982 copper penny ×2
- Propane torch or a gas stove burner
- Salt + water (electrolyte), a glass/jar, two alligator clips
- Multimeter (µA / mV)
Steps
- Oxidize: heat one copper piece until it glows dull red and goes black all over (a few min). Keep heating ~2–3 min so a thick oxide forms.
- Cool & flake: let it air-cool (don't quench). The black CuO crackles and flakes off, leaving a red-orange Cu₂O surface. Rinse gently — don't scrub the red layer off.
- Cell: stand the Cu₂O plate and a second clean copper plate in saltwater, not touching. Clip a wire to each.
- Read light: meter across the two plates. Shade it, then shine a bright light / sunlight on the Cu₂O → the voltage/current jumps. That swing = the Cu₂O photo-response. ✅ Milestone 0.
TIER 1 Solid-state memristor — Cu / CuₓO / top-contact
Now make it a dry, switchable device. The copper plate is the bottom electrode; the oxide is the switching layer; add a small top contact:
- Top contact options (low-tech first): a drop of silver conductive paint (best), a spring-loaded probe / blob of solder pressed gently, or a tiny graphite/eGaIn dot. Keep it small (~1 mm) — switching happens under the contact.
- Form & sweep: connect a current-limited supply (or Arduino DAC + series resistor ~1 kΩ to protect it). Sweep voltage
0 → +2 V → 0 → −2 V → 0 while logging current. You're looking for the pinched hysteresis loop — the memristor fingerprint — and a SET (jump to LRS) / RESET (back to HRS).
- Limit current (compliance, e.g. 1 mA) so the filament doesn't fuse permanently. This is the #1 thing that saves a cell.
TIER 2 The PHOTO-memristor — light writes the weight
Combine Tiers 0 + 1: a switching cell on the Cu₂O, with light hitting the junction.
- Light-modulated switching: run the SET sweep in the dark vs under blue/green light — the SET voltage and the LRS level shift with illumination. Light is now a second knob on the weight.
- Optical potentiation curve: hold a small read voltage; fire light pulses and plot conductance vs number of pulses. A rising, partly-retained curve = the classic synaptic-potentiation plot — the proof you built an optical synapse.
- Read / write / erase: read = tiny DC bias; potentiate = light pulses (+ optional small +V); depress/reset = −V pulse (or heat).
TIER 3 Chromatic + array + integration
- Crude color channels: Cu₂O answers blue–green (~2.0 eV); the black CuO answers red–near-IR (~1.4 eV). Two differently-oxidized cells = a 2-color "chromatic" pair — the copper cousin of your optical-chromatic-memristor neuron.
- Array: a grid of oxidized copper pads + a top-electrode comb → a small crossbar of photo-synapses.
- Integrate: drop these cells in as the light-set weights in the optical-neuron summing circuit; route with the FPGA fabric; pair with the silver/Ag₂S cells.
4 · What to measure (the proofs)
- Pinched hysteresis I-V through the origin → it's a memristor.
- Photocurrent / photovoltage swing light-on vs light-off → it's photoactive.
- SET-voltage shift under light → the two couple = photo-memristor.
- Conductance vs light-pulse count (retained) → optical synaptic potentiation.
Safety: torch + glowing metal = burns and fumes — work on a non-flammable surface, use pliers/tongs, and ventilate (metal-oxide fume). Let metal cool before handling. Current-limit every electrical test so you don't fuse (or shock) anything.
5 · Honest verdict
- Tier 0 (Cu₂O photocell) — essentially guaranteed; it's a century-old science-fair classic. Real photo-response, today, for pocket change.
- Tier 1 (CuₓO resistive switching) — real and documented (CuOₓ RRAM), but finicky: flame-grown oxide is rough and uneven, cells vary a lot, you'll kill some by over-current, and a "forming" step is often needed. Expect to make several and keep the good ones.
- Tier 2 (the coupled photo-memristor) — genuine effect and a beautiful demo, but the response will drift and won't be chip-stable. It proves the principle; it isn't a product.
- What's NOT here: a uniform, high-endurance, multi-level photo-memristor is still research-grade (needs controlled thin-film oxide, not a torch). Manage expectations: you're building a real, teaching, exciting proof — the cheapest optical synapse on Earth — not a fab-grade device.